01
Enter your average daily sales
Use the average units sold per day over the past 30–90 days. This is the baseline sell rate the forecast builds from.
Inventory Forecast Calculator
Project demand for 30–90 days with growth trend and seasonal adjustment.
Updated Reviewed by Sajid Hussain· Editor
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Last updated
June 7, 2026
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Stop guessing how much to order
Inventory forecast calculator that accounts for growth and seasonality
An inventory forecast calculator helps you project how many units you will sell over the next 30 to 90 days, so you order the right quantity — not too much and not too little. It combines your historical daily sales rate with an expected growth trend and a seasonal adjustment factor to produce a demand forecast, then calculates the safety stock and reorder point you need to stay covered through your lead time.
Most sellers either rely on gut feel or rigid spreadsheets that ignore growth and seasonality. A proper demand forecast compounds your monthly growth rate over the forecast window and multiplies by your seasonal factor — giving you a projected daily sell rate that reflects where your sales are heading, not just where they have been.
Safety stock is the buffer that absorbs demand spikes and supplier delays. Calculated as your average daily sales multiplied by your safety days buffer, it is added on top of your lead time demand to set the reorder point. The recommended order quantity fills the gap between what you will need (forecast + safety) and what you already have.
Stockout risk is surfaced in plain language: if your current stock will run out before your next shipment arrives, the calculator flags it immediately so you can act. The days-until-reorder output tells you exactly how long you have before you need to place the next purchase order.
This calculator works for any marketplace or channel — Amazon FBA, Shopify, eBay, Walmart, or wholesale. It is currency-agnostic and works in any unit: pieces, kilos, litres, or boxes.
Quick facts
- Demand forecast
- Growth-adjusted and seasonally weighted
- Safety stock
- avgDailySales × safety buffer days
- Reorder point
- Lead time demand + safety stock
- Stockout risk
- Flagged if stock runs out before lead time
- Order quantity
- Forecast + safety − current stock
- Global
- Works for any channel, currency, or product type
How it works
From daily sales to a confident purchase order
Five inputs, one purchase order recommendation — in under 30 seconds.
02
Add growth rate and seasonal factor
Set your expected monthly growth rate (0% for stable products) and a seasonal multiplier (1.5 for a 50% peak uplift, 0.7 for a slow season). These adjust the projected daily rate.
03
Set lead time and safety days
Lead time is how long it takes from placing an order to receiving stock. Safety days is the buffer you want above the lead time demand to absorb demand spikes or supplier delays.
04
Enter current stock and forecast period
How many units you have on hand today, and how many days ahead you want to forecast (30, 60, or 90 days are most common).
05
Read your demand forecast and order quantity
The calculator shows forecasted demand, safety stock, reorder point, stockout risk, and exactly how many units to order now.
Steps to use the Inventory Forecast Calculator: Enter your average daily sales, Add growth rate and seasonal factor, Set lead time and safety days, Enter current stock and forecast period, Read your demand forecast and order quantity.
Formula
The math behind the inventory forecast
Transparent formulas — no black boxes. Compound growth, seasonal adjustment, and conservative safety stock combined into one purchase order.
01
Growth multiplier (compound)
Growth Multiplier = (1 + Monthly Growth Rate ÷ 100) ^ (Forecast Days ÷ 30)
Compounds the monthly growth rate over the forecast period in 30-day months. A 10% monthly growth over 90 days = (1.10)³ = 1.331 — a 33% uplift on the baseline daily rate.
Example: 10% monthly for 90 days: (1 + 0.10)^(90÷30) = 1.10³ = 1.331
02
Forecasted daily sales
Forecasted Daily Sales = Avg Daily Sales × Growth Multiplier × Seasonal Factor
Applies growth and seasonality to the historical baseline. A seasonal factor of 1.5 means you expect 50% more demand than the baseline rate during the forecast period.
03
Forecasted demand
Forecasted Demand = round(Forecasted Daily Sales × Forecast Days)
Total units expected to sell over the forecast window. Rounded to whole units.
04
Safety stock and reorder point
Safety Stock = round(Avg Daily Sales × Safety Days) | Reorder Point = round(Avg Daily Sales × Lead Time Days + Safety Stock)
Safety stock is the minimum buffer you want on hand at all times. Reorder point is the inventory level at which you must place a new order to avoid stockout during the lead time.
05
Recommended order quantity
Recommended Order Qty = max(0, Forecasted Demand + Safety Stock − Current Stock)
The gap between what you will need (forecast + safety) and what you already have. Cannot go below zero — if you have excess stock, order nothing.
Worked example
10 units/day, 5% monthly growth, 90-day forecast
See how a real purchase order recommendation is built step by step.
Scenario
A seller moves $10.00 units per day on average and expects $5.00% monthly growth. They need to plan a $90.00-day quarter with a $30.00-day lead time and $14.00 safety days. They currently have $500.00 units in stock.
1
Step 1 · Growth multiplier
Compound 5% monthly growth over 90 days (3 months): (1 + 5/100)^(90/30) = (1.05)^3 = 1.158.
Growth multiplier: 1.158×
2
Step 2 · Forecasted daily sales
$10.00 avg daily × 1.158 growth × 1.0 seasonal = 11.58 units per day by end of quarter.
Forecasted daily: 11.58 units/day
3
Step 3 · Forecasted demand
round(11.58 × $90.00) = 1042 units over the 90-day period.
Forecasted demand: 1042 units
4
Step 4 · Safety stock and reorder point
Safety stock = round($10.00 × $14.00) = 140 units. Reorder point = round($10.00 × $30.00 + 140) = 440 units.
Reorder at: 440 units
5
Step 5 · Recommended order quantity
max(0, 1042 + 140 − $500.00) = 682 units to order.
Order: 682 units
The takeaway
Current stock of 500 units will last about 50 days at 10/day — before the 30-day lead time + 14 safety days kicks in. The seller needs to order soon and place an order for 682 units to comfortably cover the quarter.
Industry benchmarks
Forecast accuracy and lead time targets
Reference ranges for forecast accuracy and supplier lead times by sourcing region. Use these to calibrate your safety stock buffer and growth assumptions.
| Metric | Poor | Average | Good | Excellent |
|---|---|---|---|---|
Forecast accuracy (MAPE) APICS Supply Chain Management Reference 2024 | > 30% | 20–30% | 10–20% | < 10% |
Lead time — China sourcing (FBA) Flexport Ocean Freight Index 2025 | > 60 days | 45–60 days | 30–45 days | < 30 days |
Lead time — domestic / 3PL ShipBob eCommerce Fulfillment Benchmark Report 2024 | > 14 days | 7–14 days | 3–7 days | < 3 days |
Safety stock days (general ecommerce) Inventory Planner Ecommerce Survey 2024 | < 7 days | 7–14 days | 14–30 days | 30+ days (if margin permits) |
Monthly demand variability (CV) APICS Operations Management Body of Knowledge 2024 | > 50% | 30–50% | 15–30% | < 15% |
Why this calculator
Calcrux vs spreadsheets vs Inventory Planner
Most free tools calculate a flat demand number with no growth or seasonal adjustment. Paid tools like Inventory Planner charge 99–499 per month for features this calculator gives you free.
| Feature | Calcrux | Spreadsheet | Inventory Planner (paid) |
|---|---|---|---|
| Demand forecast with compound growth | Manual | ||
| Seasonal adjustment factor | Manual | ||
| Safety stock calculation | Manual | ||
| Reorder point (lead time + safety) | Manual | ||
| Stockout risk flag | |||
| Days until reorder | Manual | ||
| Works for any channel / marketplace | Ecommerce only | ||
| Free to use, no subscription | |||
| No data connection required | |||
| Aggressive growth warning |
Common mistakes
Where inventory forecasting goes wrong
Using only recent sales without accounting for trend
Why it matters
A flat daily sales average from the past 30 days ignores whether the product is growing, declining, or seasonal. Ordering based on yesterday's rate causes over- or under-ordering when the trend is significant.
Fix
Apply a monthly growth rate to compound the baseline over the forecast window. Even a 5% monthly growth compounds to 33% more demand over a quarter.
Setting safety stock too low relative to lead time
Why it matters
A safety stock of just 3 days when lead time is 30 days provides almost no protection against demand spikes. One bad week can clear the buffer and cause a stockout mid-lead-time.
Fix
Safety stock should be at least 50% of your lead time in days for normal-variability products, and 100%+ for high-variability SKUs or long-lead-time suppliers.
Forecasting for too short a window
Why it matters
A 30-day forecast may not be long enough to cover the lead time plus safety stock buffer. If your lead time is 30 days, you effectively need a 60–90-day forecast to plan a safe purchase order.
Fix
Set the forecast period to at least 2× your lead time. For a 30-day lead time, plan a 60–90-day purchase cycle.
Ignoring seasonal adjustments entirely
Why it matters
Flat averages during a slow season will cause a massive underorder for the peak that follows. Many ecommerce categories swing 50–150% between low and high season.
Fix
Use the seasonal factor input. A factor of 1.5 adds a 50% uplift to the forecast — appropriate for Q4 for many categories. Check your year-over-year data to set this correctly.
Not accounting for current stock when setting order quantity
Why it matters
Ordering the full forecasted demand without subtracting current stock leads to massively overordering. You end up with months of excess inventory and high carrying costs.
Fix
Recommended order quantity = forecasted demand + safety stock − current stock. This is the net gap, not the gross demand.
Using the same forecast for every SKU
Why it matters
A single growth rate and seasonal factor applied to every product is almost always wrong. Fast-movers, slow-movers, seasonal items, and year-round staples all need their own parameters.
Fix
Run the calculator per SKU or per product category. Group SKUs by similar demand profiles for efficiency, but do not blend fast and slow movers into one forecast.
Tips
Get more accurate inventory forecasts
Use 60–90-day sales history
A single week's data is too noisy. Use 60 to 90 days of actual sales to calculate average daily units sold, then adjust for obvious anomalies like promotions or stockouts.
Recalculate monthly, not quarterly
Monthly recalculation catches trend changes earlier. If growth is accelerating, a monthly update catches it 3× faster than a quarterly review.
Track forecast accuracy (MAPE)
Compare your forecast to actual sales each period. Mean Absolute Percentage Error (MAPE) below 20% is solid for ecommerce. If you are consistently off by 40%+, your growth or seasonal inputs need calibration.
Buffer safety days for delays
If your supplier often ships late, add extra safety days — not just the stated lead time. A 30-day lead time that slips to 45 days 20% of the time needs at least 15 days of additional buffer.
Forecast in units, not revenue
Revenue forecasts hide SKU-level stockout risk. Forecast in units per SKU so you can catch a high-velocity item running short even when overall revenue looks fine.
Pair with reorder point calculator
Use this calculator to set your purchase order quantity and the reorder point calculator to set the inventory trigger level. Together they form a complete replenishment policy.
Use cases
When sellers use the inventory forecast calculator
The Inventory Forecast Calculator works across every stage of the workflow.
Supply Chain Manager
A seller places orders every 90 days from a Chinese supplier with a 30-day lead time. They use the 90-day forecast to size each order, including the safety stock buffer, before each buying cycle.
Ecommerce Merchandising Manager
A home décor brand expects 2× demand in Q4. They set a seasonal factor of 2.0 and an aggressive growth rate to calculate how much extra stock to pre-position before October.
Amazon FBA Seller
An Amazon FBA seller needs to plan inbound shipments 3–6 weeks ahead. The reorder point and days-until-reorder outputs tell them exactly when to send the next shipment.
Ecommerce Product Manager
For a new product with no history, a seller sets the average daily sales from the first 2 weeks, applies an aggressive growth rate, and uses the forecast to size the initial bulk order.
Inventory Planner / Buyer
A seller with a product growing at 2% monthly recalculates with the realistic low growth rate — often revealing they have 6+ months of excess stock they can pause ordering on.
Multi-Channel Operations Manager
A seller splits stock between FBA and their own warehouse. They run the forecast separately for each channel using its own sales rate to determine how much to allocate to each.
Glossary
Inventory forecasting terms explained
Every important term you'll encounter in this calculator and the broader topic.
- Average daily sales
- Units sold per day on average, calculated from recent historical sales. The baseline the forecast builds from.
- Demand forecast
- A projection of how many units will sell over a future period, adjusted for growth trend and seasonal demand patterns.
- Growth multiplier
- The compound factor that converts the monthly growth rate into a total demand uplift over the forecast period. Calculated as (1 + monthly rate)^(forecast months).
- Seasonal factor
- A multiplier applied to the baseline daily sales rate to account for predictable seasonal demand shifts. A value of 1.5 means 50% more demand than the baseline.
- Lead time
- The number of days from placing a purchase order to receiving the stock. Sets the minimum inventory level at which you must reorder to avoid stockout.
- Safety stock
- A buffer quantity held above the lead time demand to absorb demand spikes, supplier delays, or forecast errors. Typically set as a number of days' sales.
- Reorder point (ROP)
- The inventory level at which you must place a new order. Equals lead time demand plus safety stock. When stock hits the ROP, order immediately.
- Recommended order quantity
- The net units to order: forecasted demand + safety stock − current stock. The exact quantity needed to cover the period without overordering.
- Stockout
- Running out of stock before the next shipment arrives. Results in lost sales, ranking penalties on Amazon, and unhappy customers.
- MAPE
- Mean Absolute Percentage Error — a standard measure of forecast accuracy. Lower is better; below 20% is solid for most ecommerce contexts.
Help & answers
Frequently asked questions
Everything you need to know about how the Inventory Forecast Calculator works.
01What is an inventory forecast calculator?
An inventory forecast calculator projects how many units you will sell over a future period — typically 30 to 90 days — by combining your historical daily sales rate with an expected growth trend and seasonal adjustment. It then calculates safety stock, reorder point, and recommended order quantity so you know exactly how much to buy.
02How do you calculate demand forecast for inventory?
Demand forecast = avg daily sales × growth multiplier × seasonal factor × forecast days. The growth multiplier compounds your monthly growth rate over the period: (1 + monthly rate)^(months). For example, 10% monthly growth over 90 days = (1.10)³ = 1.331 — a 33% uplift on the baseline daily rate.
03What is safety stock and how much should I carry?
Safety stock is the inventory buffer you hold above the lead time demand to absorb demand spikes and supplier delays. A simple rule: safety stock = average daily sales × safety days. For low-variability products with a 30-day lead time, 14 safety days is a typical starting point. High-variability products or unreliable suppliers warrant 20–30+ safety days.
04What is a reorder point and how is it calculated?
The reorder point is the inventory level at which you must place a new order to avoid running out during the lead time. Reorder point = (avg daily sales × lead time days) + safety stock. When your stock hits this level, order immediately.
05How do I account for seasonal demand in my inventory forecast?
Use the seasonal factor input. A factor of 1.5 increases projected demand by 50% above your baseline — appropriate for Q4 peak for many categories. A factor of 0.7 reduces it by 30% for a slow season. Base the factor on your year-over-year data: actual peak sales ÷ average baseline sales.
06How many days should my inventory forecast cover?
At minimum, your forecast period should equal your lead time plus safety days. For a 30-day lead time and 14 safety days, a 60–90-day forecast is appropriate. Most ecommerce businesses plan quarterly (90-day) purchase orders for overseas-sourced products.
07How do I calculate recommended order quantity?
Recommended order quantity = max(0, forecasted demand + safety stock − current stock). This is the net gap between what you will need over the period and what you already have on hand. It cannot go negative — if you have excess stock, order nothing.
08What is stockout risk and how do I avoid it?
Stockout risk occurs when your current stock will run out before the next order arrives — when days of cover < lead time days + safety days. To avoid stockouts: monitor your reorder point daily, build safety stock based on lead time variability, and reorder as soon as you hit the ROP.
09Does this inventory forecast calculator work for Amazon FBA?
Yes. Enter your average FBA daily sales, your inbound lead time (typically 3–6 weeks for China to FBA), and your safety days buffer. The calculator gives you your reorder point and recommended shipment quantity in units, which you can then send in via Seller Central.
10What monthly growth rate should I use if I am not sure?
Use 0% if the product has been stable for 3+ months with no clear trend. Use 5–10% for modest growth, 15–20% for fast-growing products with 3+ months of data. Avoid using rates above 30% unless you have solid data — high growth rates compound aggressively and can lead to massive overorders.
11How accurate are inventory forecasts?
Forecast accuracy depends on data quality and demand variability. A MAPE (mean absolute percentage error) below 20% is solid for most ecommerce businesses. High-variability products (fashion, seasonal, trending) will have higher error. The key is to recalibrate monthly using actual sales vs forecast and adjust your growth rate and safety days accordingly.
12Can I use this calculator for non-Amazon sellers?
Absolutely. The calculator works for any ecommerce channel — Shopify, eBay, Etsy, Walmart, Flipkart, or any marketplace. It is also suitable for wholesale and B2B sellers. Just enter your actual daily sales rate and supplier lead time regardless of channel.
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Category
Ecommerce Seller Operations
Subcategory
inventory operations
Availability
Global · 9 markets
Price
Free forever
Topics
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