Croston Intermittent Demand Forecast Calculator
Spare parts and slow movers sell in occasional lumps, and ordinary exponential smoothing chases each lump and then decays. Croston's method smooths demand size and demand interval separately, which gives a steadier rate. This calculator performs the update and shows the bias corrected SBA rate.
Your numbers
Smoothed size of non-zero demands before this update.
Smoothed number of periods between demands.
Interval that ended with the new demand.
Typical range 0.05 to 0.2 for intermittent items.
Your results
One update step only. In practice the estimates are initialised from history and updated only in periods with demand; periods with zero demand leave both estimates unchanged.
Forecast rate vs smoothing constant
Sweeps Smoothing constant alpha from 0.05 to 0.5, holding your other inputs fixed.
Forecast spare parts from your ERP
Optional: ask for a Croston and SBA run over your service parts history.
Optional. The tool and every export work without this. No spam.
How to use it
Enter the last stored size estimate and interval estimate for the item. When a new demand arrives, enter its quantity and the number of periods since the previous demand. Pick alpha, usually between 0.05 and 0.2, and a horizon. Store the updated size and interval as the new starting estimates, and repeat on the next demand. In periods with no demand nothing changes.
- Initialise from the first few demands or the history average
- Keep alpha low because lumps are noisy
- Track forecast at the lead time total, not per period
- Check ADI first with the CV classification tool
The formulas
When demand d occurs after q periods: z becomes z plus alpha times (d minus z), p becomes p plus alpha times (q minus p), and the forecast per period is z divided by p. Croston is known to be biased high because the expectation of a ratio is not the ratio of expectations. The Syntetos-Boylan Approximation multiplies by one minus alpha over two, which removes most of that bias and is generally preferred.
Worked example
Size 12, interval 5, a demand of 20 arrives after 3 periods, alpha 0.1. New size is 12.8, new interval is 4.8, Croston rate is 2.67 per period and the SBA rate is 2.53. Over 12 periods that is about 30 units. Compare that with a plain moving average, which would jump when the lump of 20 enters and drop again a few periods later.
Common mistakes
Applying Croston to smooth items wastes accuracy, so classify first. Measuring accuracy with MAPE fails on zeros, so use MASE, scaled error or service level outcomes instead. Setting alpha too high makes it behave like naive forecasting. And remember that the rate is an average: safety stock for intermittent items needs a lead time demand distribution, not a normal approximation.
Frequently Asked Questions
When should I use Croston instead of exponential smoothing?
When many periods are zero, roughly ADI above 1.32. For smooth items with demand nearly every period, plain exponential smoothing is simpler and usually just as accurate.
What is the SBA correction?
The Syntetos-Boylan Approximation multiplies the Croston rate by one minus alpha over two. It reduces the upward bias in Croston's estimate and is widely used for spare parts.
Does a zero period update the forecast?
No. In the classical method both estimates are only updated when a non-zero demand occurs. The interval count simply keeps growing until the next demand.
How do I measure accuracy for intermittent items?
Avoid MAPE because actuals are often zero. Use scaled errors such as MASE, bias, or judge by the stock outcomes: fill rate and inventory held.
Netray can compute Croston or SBA forecasts for your spare parts directly from ERP transaction history and write suggestions back for planner approval.
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