Demand Forecast Error (MAPE) Calculator

Per-period APE table

How MAPE helps forecast evaluation

Mean Absolute Percentage Error (MAPE) is a common way to evaluate forecast accuracy because it expresses errors as a percentage of actual demand. This makes it easy to compare accuracy across products or time periods with different demand levels. MAPE is calculated as the average of absolute percentage error (APE) for each period, where APE is |Actual − Forecast| ÷ Actual.

This calculator accepts actual and forecast series as comma- or newline-separated values. The lists must be the same length, and each value must be numeric. Periods where actual demand is zero are excluded from MAPE because the percentage error is undefined. The tool reports how many valid periods were used so you can understand whether zero-demand periods affected the result.

The per-period APE table provides transparency. It lists actuals, forecasts, absolute errors, and APE for each period, making it easier to identify outliers or seasonal issues. Large APE values often indicate a mismatch in trend or a one-time event, and they can skew the MAPE. Reviewing the table helps determine whether the error is systematic or isolated.

While MAPE is widely used, it has limitations. It is biased toward low-volume periods because a small absolute error can translate into a high percentage error when actual demand is low. If your data has frequent zeros or low values, consider complementary metrics such as MAE or RMSE. Still, MAPE remains a practical standard for many supply chain planning teams because it is intuitive and easy to communicate.

Use this calculator when evaluating forecast models, monitoring accuracy over time, or comparing baseline versus improved forecasts. All calculations run in your browser, so demand data stays private.

Formula

APE: |Actual − Forecast| ÷ Actual

MAPE: Average of APE across valid periods

Example calculation

If actual demand is [120, 135, 150] and forecast is [125, 130, 155], the APEs are 5/120 = 4.17%, 5/135 = 3.70%, and 5/150 = 3.33%. The MAPE is the average: (4.17% + 3.70% + 3.33%) ÷ 3 = 3.73%.

FAQs

Why exclude periods with actual = 0?

Percentage error is undefined when actual demand is zero, so those periods are removed from MAPE.

Can I use weekly or monthly data?

Yes. The calculator treats each entry as one period regardless of time scale.

What if lengths do not match?

The tool will show an error and will not compute MAPE until the series lengths match.

Is MAPE the best metric?

MAPE is popular, but you may want to also track MAE or RMSE for a more complete view.

Is this calculator private?

Yes. All calculations run locally in your browser.

How it works

This calculator parses series inputs, computes per-period APE, and averages valid periods for MAPE. All computation is client-side for privacy.

Disclaimer

MAPE is sensitive to low actual values and outliers. Use additional metrics for a full accuracy review.