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Prophet Trend Estimate Calculator

Work out prophet trend estimate instantly with clear inputs, formula shown and shareable results.

About Prophet Trend Estimate Calculator

Prophet models trend as a piecewise linear function: growth is constant between changepoints and shifts by a fitted delta at each one. Projecting forward means taking the growth rate of the final segment and extrapolating it. That is why Prophet forecasts are so sensitive to a changepoint detected near the end of the series — the last segment's slope sets the entire future path.

Formula

Piecewise linear trend

g(t) = base + (growth x multiplier) x days since changepoint; counterfactual = base + growth x days

Worked example

With the default values below, Prophet Trend Estimate Calculator returns Trend forecast of 1,607.50. Change any input above and every figure here is recalculated in your browser.

Inputs used

Level at the changepoint
1,000
Growth per day before the changepoint
4.5
Growth multiplier after the changepointaccepts 0.01–10
1.5
Days after the changepointminimum 1
90

Results

Trend forecast
1,607.50
Growth rate after the changepoint
6.750 per day
Total change over the horizon
607.50
Difference from the unchanged trend
202.50

Step by step

  1. 1Values used
  2. 2Piecewise linear trend
  3. 3Trend forecast= 1,607.50
  4. 4Growth rate after the changepoint= 6.750 per day
  5. 5Total change over the horizon= 607.50
  6. 6Difference from the unchanged trend= 202.50

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, fees or charges or compounding frequency 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

Why does Prophet sometimes forecast explosive growth?

A changepoint fitted to the last few noisy observations can produce a steep final slope. Reduce changepoint_prior_scale or restrict changepoints to the first 80 percent of the history.

When should logistic growth be used instead?

Whenever there is a natural ceiling such as market size or physical capacity. Linear growth will otherwise forecast past what is possible.

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Prophet Trend Estimate Calculator – Calculate Trend forecast