How the seasonality index works
Every FBA product has months where it sells faster and months where it slows down. A seasonality index quantifies that pattern as a multiplier for each month of the year. You calculate it from at least one full year of sales data (two years is better for smoothing anomalies).
The index is a ratio: each month’s average daily sales divided by the overall average daily sales across all months. A month that sells exactly at the annual average gets an index of 1.0. A month that sells 40% above average gets 1.4. A month that sells 25% below average gets 0.75.
Once you have the 12 monthly indices, you multiply your baseline demand forecast by the index for whatever month the inventory will be selling during. This is what separates a flat forecast from one that actually matches your business rhythm.
Seasonality index formula
Example: a $38 holiday gift product
You sell a bamboo cutting board set at $38 ASP. It’s a popular gift item that surges in Q4. Here’s your 2-year average daily sales by month:
| Month | Avg daily units | Seasonality index |
|---|---|---|
| Jan | 15 | 0.60 |
| Feb | 14 | 0.56 |
| Mar | 18 | 0.72 |
| Apr | 20 | 0.80 |
| May | 22 | 0.88 |
| Jun | 24 | 0.96 |
| Jul | 21 | 0.84 |
| Aug | 23 | 0.92 |
| Sep | 28 | 1.12 |
| Oct | 35 | 1.40 |
| Nov | 48 | 1.92 |
| Dec | 32 | 1.28 |
Overall average daily sales: (15+14+18+20+22+24+21+23+28+35+48+32) / 12 = 25.0 units/day
October index: 35 / 25 = 1.40
January index: 15 / 25 = 0.60
Now apply it. Your baseline forecast (from exponential smoothing) says 25 units/day. For a PO landing in November with a 75-day lead time, you’d place the order in late August. The inventory needs to cover November and December selling:
November adjusted forecast: 25 × 1.92 = 48 units/day × 30 days = 1,440 units
Without the seasonality index, you’d order 25 × 30 = 750 units for November and stockout halfway through the month. The index prevents a $27,360 revenue gap (690 units × $38 + lost organic rank).
FBA-specific considerations
Amazon’s seasonal storage surcharges add another layer. From October through December, FBA storage fees increase significantly, and aged inventory surcharges still apply. Your seasonality index tells you to send more inventory for Q4, but Amazon’s fee structure punishes you for sending it too early. The timing math matters: ship enough to cover peak demand but land it close to when it sells, typically arriving at FBA by mid-September for holiday products.
Also watch for Amazon’s receiving delays during peak. If your shipment takes 14 days to check in during October instead of the usual 5, your effective lead time extends by 9 days. Factor this into the forecast window when placing your Q4 PO. Amazon’s FBA inventory storage fees page details the current seasonal surcharge schedule.
Common mistakes
- Using only one year of data. A single year can be distorted by a one-time event (a viral TikTok, a competitor stockout, a bad review period). Two years of data smooths these out and produces a more reliable index.
- Applying the same index to every SKU. A kitchen gadget and a winter coat have completely different seasonal curves. Calculate the index per SKU or at minimum per product category.
- Forgetting to adjust for stockouts in historical data. If you were out of stock for two weeks in November last year, your November average is artificially low and the index will understate Q4 demand. Backfill stockout periods before calculating.