Holt Winters Forecast Calculator

Work out holt winters forecast instantly with clear inputs, formula shown and shareable results.

About Holt Winters Forecast Calculator

Holt-Winters carries three states: level, trend and a seasonal index per period. The multiplicative forecast is (level + h x trend) multiplied by the seasonal factor for the target period, so seasonal swings scale with the level — the right assumption when December is 15 percent above trend rather than a fixed number of units above it.

Formula

Multiplicative Holt-Winters

F(t+h) = (L(t) + h x T(t)) x S(t+h-m)

L: Level
T: Trend per period
S: Seasonal factor
m: Season length

Worked example

With the default values below, Holt Winters Forecast Calculator returns Multiplicative Holt-Winters forecast of 253.000. Change any input above and every figure here is recalculated in your browser.

Inputs used

Current level: 200
Current trend per period: 5
Seasonal factor for the target periodminimum 0.01: 1.15
Periods aheadminimum 1: 4

Results

Multiplicative Holt-Winters forecast: 253.000
Level plus trend before seasonality: 220.000
Contribution of the trend over the horizon: 20.000
Adjustment from the seasonal factor: 33.000

Step by step

1Values used
2Multiplicative Holt-Winters
3Multiplicative Holt-Winters forecast= 253.000
4Level plus trend before seasonality= 220.000
5Contribution of the trend over the horizon= 20.000
6Adjustment from the seasonal factor= 33.000

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or fees or charges into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Additive or multiplicative seasonality?

Multiplicative when the seasonal swing grows with the level, which is typical of sales. Additive when the swing is a roughly constant amount, common in temperature or headcount series.

Why does the forecast drift so far at long horizons?

The trend term is extrapolated linearly, so errors compound with h. Damped-trend variants multiply the trend by a factor below one per step and forecast much more conservatively.