How to make a heatmap
- Enter one row per cell with its column category (X), row category (Y) and value. Paste long-format data from a spreadsheet or pivot table.
- Categories appear in the order you first use them. Sort your rows first if you want a specific order.
- Choose a color scale in Customize → Colors & style. Use sequential for magnitudes and diverging when values sit above and below a midpoint.
- Toggle values in cells, adjust the cell gap, and optionally fix the color scale range.
- Download your heatmap.
Data format
| Column | Type | Notes |
|---|---|---|
| Column (X) | Text | Required |
| Row (Y) | Text | Required |
| Value | Number | Required |
Up to 2,500 rows. Long format: one row per cell with its column category, row category and value. Categories appear in the order they are first used. Missing combinations are shown as empty (hatched) cells — they are not treated as zero.
Customization options
- Six color scales, including a perceptually ordered viridis-style scale and a diverging red–blue scale, with a reverse option.
- Values shown in cells in automatically contrasting text colors.
- Fixed color range to keep several heatmaps comparable.
- Hatched empty cells for missing data.
- Color legend showing the min-to-max gradient.
Downloading and exporting
- PNG: best for slides, documents and the web. It supports transparent backgrounds, and you can choose standard, 2×, 4× or slide-sized (1920 × 1080) output.
- JPEG: a smaller file with a solid background.
- SVG: a true vector file. It stays sharp at any size and can be edited in Illustrator, Inkscape or Figma.
- PDF: vector PDF sized to the chart, A4 or US Letter. If your labels use characters outside the standard PDF fonts (for example Chinese, Arabic or emoji), the PDF uses a high-resolution image instead and tells you.
- CSV and .json: your data, or the whole chart with its settings, to re-open later.
When to use a heatmap
- Finding patterns in two-way tables: activity by day and hour, sales by product and month, correlation matrices.
- Large grids where a table of numbers is overwhelming.
When to choose a different chart
- Precise values matter more than patterns. Show a table instead, or turn on cell values.
- You have only one category dimension. Use a bar graph.
- Readers need to compare similar shades. Color is an imprecise channel, so consider a bubble chart or grouped bars.
Common mistakes to avoid
Rainbow color scales
Rainbow palettes create false boundaries where hues change. Use a single-hue or viridis-style scale for ordered data.
Diverging scale with no meaningful midpoint
Use the diverging scale only when the middle value (for example 0 or a target) is meaningful, and set symmetric min and max values.
Treating blanks as zero
A missing measurement is not a zero. This tool hatches empty cells so they cannot be mistaken for low values.
Practical examples
Café traffic
Average customers by weekday and hour, showing the Friday-evening rush.
Correlation matrix
Pairwise correlations between six variables, with the diverging scale fixed from −1 to 1.
Website engagement
Pages per session by traffic source and device type.
Long format vs. a grid
Spreadsheets often store heatmap data as a grid, with rows down the side and columns across the top. This tool uses the “long” format, with one row per cell, because it handles missing combinations and any number of categories without ambiguity. To convert a grid in Excel or Google Sheets, unpivot it (in Google Sheets with a FLATTEN formula, or in Excel with Power Query → Unpivot Columns), then paste the three resulting columns.
Frequently asked questions
Can I paste a matrix (grid) directly?
Not directly. The tool expects one row per cell (X, Y, value). Unpivot your grid in Excel or Google Sheets first; the explanation above shows how.
What happens if a combination appears twice?
Only the first value is used, and an error tells you which row was ignored.
How many cells can a heatmap have?
Up to 2,500 cells and up to 100 categories on each axis.
Can I reverse the colors?
Yes. Tick “Reverse color scale” in Colors & style.
Is the viridis-style scale colorblind-friendly?
Viridis-style scales were designed to stay ordered for common forms of color-vision deficiency and when printed in grayscale.
Your data stays private
Charts are drawn by your browser. Your data isn’t uploaded to our servers; it is kept only in this browser’s storage, for autosave and saved charts, until you clear it. See our privacy policy.