How much stock do your subscriptions need, and when does it run out?
How much stock will your subscriptions need?
Enter your own numbers for one product. The calculator starts on the worked example further down this page.
All results are estimates from your inputs, not figures from your store.
How does the inventory forecast work?
- Deliveries per week = deliveries per subscription a month × 12 ÷ 52. Enter 4.33 deliveries a month for every week, 2.17 for every 2 weeks and 1 for every month, the conversion of the MRR glossary page. They make 1, 0.5 and about 0.23 deliveries a week.
- Units needed in a week = subscriptions at the start of the week × deliveries per week × units per delivery.
- Subscriptions next week = this week’s subscriptions × (1 − monthly churn rate)^(12 ÷ 52) + new subscriptions per month × 12 ÷ 52. The monthly churn rate and the new subscriptions are spread over the weeks of a month.
- Stock left = current stock − the units needed so far. The stock runs out in the first week where it drops below zero.
- Units short = units needed over the forecast − current stock, when that’s more than zero.
The forecast spreads deliveries evenly over each interval.
When do 2,500 units run out?
Example (made-up numbers): 400 active subscriptions get 2 units of a product twice a month. 40 new subscriptions start each month, 4% cancel each month, and 2,500 units are in stock. The forecast covers 12 weeks.
| Week | Units needed | Total so far | Stock left |
|---|---|---|---|
| 1 | 369 | 369 | 2,131 |
| 2 | 374 | 744 | 1,756 |
| 3 | 379 | 1,123 | 1,377 |
| 4 | 384 | 1,507 | 993 |
| 5 | 389 | 1,896 | 604 |
| 6 | 394 | 2,290 | 210 |
| 7 | 399 | 2,689 | −189 |
| … | … | … | … |
| 12 | 422 | 4,754 | −2,254 |
Twice a month is 2 × 12 ÷ 52 ≈ 0.4615 deliveries a week, so week 1 needs 400 × 0.4615 × 2 ≈ 369 units. The stock covers six weeks and runs out in week 7; over 12 weeks, 2,254 units are missing. Subscriptions grow from 400 to about 462, because 40 new ones a month outweigh 4% churn.
What does the forecast leave out?
- Billing days. If most subscriptions bill on the same day, a single day needs far more than a week’s average. Plan by billing date where you can.
- Skips, pauses and restocks. A skipped or paused delivery needs no stock, so real demand can come in lower, and the stock you reorder isn’t counted.
- One-time orders. One-time buyers order from the same stock. Without a reservation, they can take units your subscribers need on billing day.
How does Zubs plan and reserve subscription stock?
The Inventory Planner in Zubs forecasts the units each product needs from your real subscriptions, next to your stock in Shopify. It can also hold that stock before each billing date, so one-time orders can’t sell it first, and it shows which products will run short.
Bottom line
Your subscriptions need subscriptions × deliveries per week × units per delivery each week, growing with new sign-ups and shrinking with churn. Compare the running total with your stock to see the week it runs out, and reorder before then or hold the stock for your subscribers.
Frequently asked questions
- Why does the forecast step week by week?
- Because subscriptions change every week. New ones sign up and some cancel, so each week's demand starts from that week's subscriptions instead of today's.
- What if all my subscriptions bill on the same day?
- Then demand comes in batches, not evenly every week. The total over several weeks still holds, but a single billing day can need much more than one week's average.
- Does the forecast count one-time orders?
- No. It forecasts subscription deliveries only. One-time buyers draw on the same stock, so keep a buffer for them or reserve the subscription stock.
- Is my data stored or sent anywhere?
- No. The calculation runs in your browser, and nothing you enter leaves the page.