Free forecast accuracy calculator

This free forecast accuracy calculator works out WAPE, MAPE, bias and forecast accuracy from your own numbers. Paste your forecast and actual columns from Excel, one item per line. An item name in front is optional. It runs in your browser.

Forecast accuracy calculator

Forecast and actual, one item per line

Estimates for planning only. Results depend entirely on the numbers you enter. Nothing you enter is stored or sent.

The forecast accuracy formulas

WAPE = sum of |forecast − actual| ÷ sum of actual
Forecast accuracy = 1 − WAPE
MAPE = average of (|forecast − actual| ÷ actual)
Bias = (sum of forecast − sum of actual) ÷ sum of actual

A worked example

Five items total 1,000 units forecast and 1,000 actual, with absolute errors adding up to 140. WAPE is 14.0%, so accuracy is 86.0%. MAPE is 31.7%, because one tiny item missed by 100%. Bias is 0.0%, because the highs and lows cancel out. Use the example button above to see it. For more detail, read our guide to the forecast accuracy formula.

Frequently asked questions

What is a good MAPE?

There is no single good score, because it depends on the item. Steady, high-volume items can be forecast closely, and erratic, low-volume ones cannot. Compare your result with a simple method such as last year’s sales.

Why are WAPE and MAPE different?

WAPE weights items by volume. MAPE treats every item equally, so a small item with a large percentage miss can pull it up.

What about items with zero sales?

MAPE cannot divide by zero, so this calculator leaves those lines out of MAPE and says so. WAPE and bias still include them.

Want the error for every item, with the forecast method tested on your own history? The Optimization Lab shows it on the Planner and Growth plans.

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