FORECAST Function (Gnumeric)

Gnumeric’s FORECAST estimates a y-value for a supplied x-value by fitting a straight line to paired numeric observations. The fitted line has the form:

[\widehat{y}=a+bx,\qquad b=\frac{\sum_i(x_i-\bar{x})(y_i-\bar{y})}{\sum_i(x_i-\bar{x})^2},\quad a=\bar{y}-b\bar{x}.]

Here b is the slope and a is the intercept. The function is appropriate for an introductory straight-line forecast, not a guarantee of causal relationships or real-world future performance. Gnumeric’s Statistics manual explicitly documents the function as simple linear regression; GNOME’s source computes a linear regression and applies its intercept and slope to x. [S1, S3]

=FORECAST(x,known_ys,known_xs)

The order is forecast-x, known-y, known-x. Reversing the last two arguments fits a different regression. This is also the order Gnumeric’s official manual and 1.12.62 upstream code specify. There are no optional parameters. [S1, S3]

Enter this table in a blank Gnumeric worksheet; column A holds period and column B holds observed units sold.

Put 6 in D2 as the month to predict, and type into E2:

=FORECAST(D2,B2:B6,A2:A6)

Independently calculated expected result: 13. The points follow y = 1 + 2x exactly; month 6 therefore gives 1 + 2*6 = 13. Verify the fit separately with:

=SLOPE(B2:B6,A2:A6)
=INTERCEPT(B2:B6,A2:A6)

The first formula should give 2, the second 1, based on independent arithmetic. Reconstruct the forecast explicitly:

=INTERCEPT(B2:B6,A2:A6)+SLOPE(B2:B6,A2:A6)*D2

It also gives 13 mathematically. These are documentation-supported predictions, not the results of a native Gnumeric session. See SLOPE and INTERCEPT for related canonical pages. [S1]

Overwrite B2:B6 with 2, 4, 5, 4, 5 (keep A2:A6 as 1, 2, 3, 4, 5). Enter:

=FORECAST(6,B2:B6,A2:A6)

Independent result: 5.8. The fitted slope is 0.6, the intercept is 2.2, and therefore the prediction at x=6 is 2.2 + 0.6*6 = 5.8. The input values, fitted values and residuals do not lie on a perfect line, making this a more realistic explanation than an exact-fit-only example.

Contrast the forecast for an x inside the measured range:

=FORECAST(3,B2:B6,A2:A6)

Independent result: 4. Predicting x=3 is interpolation; x=6 is extrapolation. Extrapolation assumes the straight-line pattern continues beyond the measured range, an assumption the worksheet cannot guarantee. For a report, pair FORECAST with RSQ to inspect fit quality and STEYX for regression standard error, bearing in mind that these diagnostics do not establish causality. [S1]

Gnumeric’s upstream help supplies y observations 11.4, 17.3, 21.3, 25.9, 40.1 and x observations 23.2, 25.8, 29.9, 33.5, 42.7. Put the y-values in A1:A5 and matching x-values in B1:B5; then enter:

=FORECAST(7,A1:A5,B1:B5)

The source documents a result of approximately −10.859397661; an independent least-squares calculation gives −10.8593976609. These agree at the reported precision. The negative result is a consequence of extrapolating far below the measured x range; it is not automatically a physically plausible forecast for every business use case. [S3]

Exact-release source note: The official 1.12.62 source archive was checksum-verified. In src/collect.c, these paired-range functions receive GNM_ERROR_VALUE as the fallback: no accepted pairs, unequal collected lengths and failed numeric callbacks therefore follow a #VALUE! source path. Some manual entries instead name #N/A or #DIV/0 for empty/mismatched/constant inputs. Those manual tokens below are documentation claims, not confirmed runtime outcomes; the actual installed build was not executed.

Do not replace inconvenient x values with dummy numbers to suppress errors. Avoid statements that FORECAST necessarily predicts time: its x-axis can be any numeric explanatory variable. It is a statistical estimate, not a time-series seasonal forecasting model. [S1, S3]

Keep the x and y data in A2:A6 and B2:B6. Enter proposed x-values in D2:D5, e.g. 6, 7, 8, 9. Put in E2:

=FORECAST(D2,$B$2:$B$6,$A$2:$A$6)

Copy E2 through E5. The absolute $ references hold the training data fixed, while the proposed x changes by row. Using the exact-fit first example, the independently expected output sequence is 13, 15, 17, 19. The dollar-sign notation is described in Gnumeric’s official formula manual. [S5]

• SLOPE — fitted line slope.
• INTERCEPT — fitted line intercept.
• RSQ — square of correlation; a simple fit statistic.
• STEYX — residual standard error.
• TREND — related regression prediction function; read its own syntax rather than assuming identical arguments.

• Documentation target: Gnumeric 1.12, cross-checked with the 1.12.62 release (released September 20, 2026; GNOME download directory posted September 21). This is documentation/source checking, not an on-device runtime certification. [S1–S4]
• The function has a dedicated entry in Gnumeric’s Statistics reference and an implementation registered Complete with Basic upstream tests in the public catalogue. Registered means the upstream distribution advertises the function; it does not prove that the individual examples here were run successfully on your OS, distribution, locale or plugin configuration. [S1–S3]
• Gnumeric’s manual writes English-function examples with an equals sign, uses commas between arguments, and states function names are case-insensitive. Its locale and number formats can vary; these examples use an English-language, dot-decimal configuration. Verify localized input syntax and displayed separators inside your installed application rather than assuming another spreadsheet’s settings. [S5–S6]
• Gnumeric marks this function Microsoft Excel compatible in its own function help. That is a vendor statement about the documented function, not proof of identical results for all imported files, spreadsheet programs, edge cases or historical editions. ODF/other-spreadsheet behavior has not been used to infer undocumented Gnumeric behavior. [S1]

1. S1 — Official Gnumeric 1.12 Statistics function manual. Relevant lines in current published HTML: 819–848; defines function name, exact argument order, documented errors and vendor Excel-compatibility statement. Publication/update date of the HTML page is not specified; access checked 2026-10-13.
2. S2 — Official public function catalogue. Statistics listing registers forecast as Implementation: Complete; Testing: Basic. This is upstream catalogue status, not a test performed for this guide; access checked 2026-10-13.
3. S3 — Official Gnumeric 1.12.62 source archive. plugins/fn-stat/functions.c and src/collect.c were inspected after checksum verification. This is exact-release source evidence, not a native execution.
4. S4 — Gnumeric release 1.12.62 announcement and GNOME official source-release index. The Gnumeric source ChangeLog records 2026-09-20, and GNOME’s source index timestamps the tarball 2026-09-21. Release availability does not prove that this guide’s examples were run on any installed OS.
5. S5 — Gnumeric formula entry/type/reference manual, including =, commas, relative/absolute references and typical error names; checked 2026-10-13.
6. S6 — Gnumeric language/locale manual, locale-controlled numeric representation; checked 2026-10-13.

Validation limits: Exact official 1.12.62 source was checksum-verified and inspected, and all displayed numbers independently recalculated. No native Gnumeric execution, screenshots, locale regression or workbook round-trip tests were performed.