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FORECAST Function (LibreOffice Calc)

Math Beginner LibreOffice Calc Introduction not established

The FORECAST function in LibreOffice Calc predicts a single Y-value using linear regression. This guide explains syntax, examples, regression logic, errors, and best practices.

Verification status: Consult the documentation, examples and any stated test scope below. How formula examples are checked

Availability and version history

  • Introduced: Documented by LibreOffice 26.8.1.1; original introduction not established.
  • Availability: Supported in LibreOffice 26.8.1.1, verified against its function registry. This identifies a supported release, not a final working version.
  • Scope: Desktop Calc, using the English function name. Availability in other spreadsheet applications is separate.
  • Checked: 2026-10-14 — function reference; versioned support. Documentation/source check; not a new native test. Existing example-specific test claims below retain their stated scope.
The **FORECAST** function predicts a **single Y-value** based on a linear regression model. It is the simplest way to generate a one-off prediction without using TREND or LINEST.

What the FORECAST Function Does

  • Predicts a single Y-value using linear regression
  • Uses the same model as TREND and LINEST
  • Ideal for simple predictions and quick estimates
  • Works with numeric X/Y pairs
  • Works across sheets

FORECAST is essentially:

y = m*x + b

Where m and b come from least-squares regression.

Syntax

FORECAST(x; known_y; known_x)

Where:

  • x — the X-value to predict for
  • known_y — dependent variable (Y values)
  • known_x — independent variable (X values)
X and Y ranges must be the same size.

Basic Examples

Predict Y for X = 11

=FORECAST(11; B1:B10; A1:A10)

Predict Y for a value stored in a cell

=FORECAST(D1; B1:B100; A1:A100)

Forecast across sheets

=FORECAST(2025; Sheet1.B1:B50; Sheet2.A1:A50)

Forecast using dates

=FORECAST(DATE(2026;1;1); Sales; Dates)

Advanced Examples

Manual equivalent using SLOPE and INTERCEPT

=SLOPE(B1:B10; A1:A10) * 11 + INTERCEPT(B1:B10; A1:A10)

Forecast ignoring errors

=FORECAST(11; IF(ISNUMBER(B1:B10); B1:B10); A1:A10)

(Confirm with Ctrl+Shift+Enter in older Calc.)

Forecast using filtered (visible) data only

Use SUBTOTAL helper column to filter X/Y before passing to FORECAST.

Forecast with outlier removal

=FORECAST(11; FILTER(B1:B100; B1:B100<1000); FILTER(A1:A100; B1:B100<1000))

Forecast for time-series with irregular spacing

=FORECAST(D2; B$1:B$100; A$1:A$100)

Forecast using named ranges

=FORECAST(NewX; KnownY; KnownX)

How FORECAST Calculates Predictions

FORECAST uses the same regression model as LINEST:

  1. Compute slope (m):

    m = COVAR(known_x; known_y) / VAR(known_x)

  2. Compute intercept (b):

    b = AVERAGE(known_y) - m * AVERAGE(known_x)

  3. Predict:

    y = m * x + b

FORECAST is simply a convenience wrapper around these calculations.

Common Errors and Fixes

Err:502 — Invalid argument

Occurs when:

  • X and Y ranges have mismatched lengths
  • Non-numeric text included
  • x is non-numeric

Err:504 — Parameter error

Occurs when:

  • Semicolons are incorrect
  • Range references malformed

FORECAST returns unexpected value

Possible causes:

  • Relationship is not linear
  • Outliers distort regression
  • X-values not aligned with Y-values

FORECAST differs from TREND

TREND returns multiple predictions; FORECAST returns one.

Best Practices

  • Use FORECAST for single-value predictions
  • Use TREND for multi-value forecasting
  • Use LINEST when you need full regression statistics
  • Remove outliers before modeling
  • Plot your data to confirm linearity
  • Use named ranges for cleaner formulas
FORECAST is perfect when you need a quick, single prediction — no arrays, no complexity, just a clean linear estimate.