What is a contour interval?

A contour interval is the fixed vertical distance (in meters or feet) between two adjacent contour lines on a topographic map. Every line represents a specific elevation, and all points on that line share the same height above sea level.

How do I count contour intervals vs. contour lines?

The number of contour intervals equals the number of contour lines minus one. If a map section shows 6 contour lines, there are 5 intervals between them. This calculator asks for intervals, not lines, so subtract 1 from your line count before entering.

What is a typical contour interval on USGS topo maps?

USGS 7.5-minute quadrangle maps (1:24,000 scale) use intervals ranging from 5 ft (gentle terrain) to 80 ft (steep mountains). The most common is 40 ft. Metric USGS maps typically use 10 m or 20 m intervals.

What does a small contour interval indicate about terrain?

A small contour interval means the map is showing gentle terrain or fine elevation detail. When lines are crowded together with a small interval, the slope is very steep. When lines are far apart, the terrain is nearly flat.

Can I use this for index contours?

Index contours are every fifth contour line and are drawn bolder for readability. The contour interval itself does not change — index contours simply provide labeled reference points. The formula and this calculator apply equally to both index and intermediate contours.

Contour Interval Calculator

Name: Contour Interval Calculator
Availability: InStock
Author: Nham Vu

Enter the total elevation change and the number of contour lines to instantly calculate the contour interval for any topographic map.

Contour Interval Inputs

Total Elevation Change

Difference between highest and lowest contour line

Number of Contour Intervals

Spaces between lines = line count − 1

Common Map Presets

Result

Enter values and click Calculate to see the result.

Summary

Enter the total elevation change and the number of contour lines to instantly calculate the contour interval for any topographic map.

How it works

Enter the total elevation change (the difference between the highest and lowest contour lines on the map).
Enter the number of contour intervals — this is the count of spaces between lines, not the number of lines themselves.
Select your preferred unit (meters or feet).
Click Calculate to get the contour interval — the vertical distance between each pair of adjacent contour lines.
Use the result to interpret slope steepness: lines spaced at a small interval mean steep terrain; large intervals mean gentle slopes.

Use cases

Determine the contour interval of a printed topo map when the legend is missing or unclear.
Check contour interval consistency when merging elevation data from multiple map sheets.
Teach students the relationship between contour line count and terrain relief.
Verify GIS-generated contour outputs against known elevation range and line count.
Plan hiking routes by converting contour intervals to expected elevation gain per map segment.
Cross-check survey data: known elevation span and field-counted lines should match the published interval.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu