Function Problems

Fix Incorrect Sixth-Degree Polynomial Trendline and LINEST Results in Excel

Rana Garcia Sep 28, 2026 868 views

Question details

The user is experiencing incorrect or unexpected equations and LINEST function results when attempting to calculate a sixth-degree polynomial trendline in Excel.

How to Fix Incorrect Sixth-Degree Polynomial Trendline and LINEST Results in Excel

Product: Microsoft Excel
Device & OS: not provided
Scenario: Performing complex data regression analysis using high-degree polynomial trendlines and the LINEST function.
Observed behavior: Excel displays inaccurate equations and LINEST outputs for sixth-degree polynomials, despite lower-degree calculations functioning normally.

Before you start

Before modifying your regression models, ensure there are no empty cells or text values hidden in your raw data range, as these can drastically skew LINEST calculations and disrupt trendline rendering.

Solution 1Recommended

Increase Coefficient Decimal Places for Polynomial Trendline Labels

Trendline equations for high-degree polynomials often calculate incorrectly when used manually because Excel heavily truncates the coefficients shown in the chart label. Increasing decimal precision resolves this.

In high-degree polynomials (like a sixth-degree model), even a microscopic rounding error in a coefficient causes massive discrepancies when calculating predicted Y values. By default, Excel chart labels display aggressively rounded coefficients.

Select the trendline equation

Click on the trendline equation label directly within your Excel chart to highlight the text box.

Open Format Trendline Label pane

Right-click the highlighted equation and select 'Format Trendline Label' from the context menu to open the formatting sidebar.

Increase decimal precision

In the formatting pane, change the Category dropdown from 'General' to 'Number' or 'Scientific', and increase the Decimal places to 10 or 15 to view the exact mathematical coefficients.

Increase Coefficient Decimal Places for Polynomial Trendline Labels

Accurate Manual Calculation: Using these precise coefficients in your worksheet formulas will yield the correct curve matching the visual trendline.

Solution 2

Fit the Data Using an Alternative COS Function Model

If LINEST continues to fail or the dataset inherently limits polynomial accuracy, using a trigonometric COS function model can provide a better fit and more reliable R-squared values.

Free Microsoft Office alternative

Try WPS Office for Seamless Data Analysis

If you frequently encounter limitations or complex calculation quirks in Microsoft Excel, consider trying WPS Office. It provides a lightweight, highly compatible, and free alternative equipped with powerful spreadsheet capabilities.

1. Download WPS Office: Visit the official WPS website to download and install the free software suite.
2. Open your data workbook: Launch WPS Spreadsheet and open your existing Excel (.xlsx) file to continue your data analysis without losing any formatting or charts.

Fully compatible with Microsoft Excel (.xlsx, .xls, .csv) formats and complex formulas.Reliable charting and data analysis tools for high-degree statistical workflows.Lightweight architecture that loads massive statistical workbooks quickly.Familiar user interface ensuring a zero-learning-curve migration.

microsoft office alternative - wps office

Frequently Asked Questions

Why does my Excel trendline equation calculate completely different Y values compared to the chart?

This happens because Excel rounds the coefficients shown in the trendline label for visual simplicity. For high-degree polynomials, substituting X into these rounded coefficients multiplies the rounding error exponentially. You must format the trendline label to display up to 15 decimal places for accurate mathematical results.

What is the highest degree polynomial trendline Excel supports?

Excel supports polynomial trendlines up to the sixth degree (Order 6). If your data requires higher-order analysis, you will need to utilize specialized statistical software or alternative mathematical models, such as trigonometric functions or custom VBA scripts.

Can I use the LINEST function to calculate polynomial coefficients directly instead of relying on the chart?

Yes. You can calculate polynomial coefficients using LINEST by modifying the known_x's argument into a horizontal array. For example, for a 3rd-degree polynomial, you would use =LINEST(Y_range, X_range^{1,2,3}). Ensure you enter it as an array formula (Ctrl+Shift+Enter) if you are using older versions of Excel.

Excel & Data Problems