How do you find the contour interval on a topographic map?

Pick two labeled index contours, subtract their elevations, and divide by the number of spaces between them (unlabeled lines between them + 1). Example: 600 ft and 500 ft index contours with 4 lines between them give 100 ÷ 5 = 20 ft. The interval is also printed in the map margin, usually below the scale bar.

What is a contour interval?

A contour interval is the fixed vertical distance between two adjacent contour lines on a topographic map. Every line joins points of equal elevation, so walking from one line to the next always changes your elevation by exactly one interval.

What is an index contour?

An index contour is a thicker contour line labeled with its elevation. On USGS maps it is every fifth line, so four thinner intermediate lines sit between index contours and the difference between neighboring index contours is five times the contour interval.

How do I count contour intervals vs. contour lines?

Intervals are the spaces between lines. Between two counted lines the number of intervals is the line count minus one; between two index contours with n unlabeled lines between them it is n + 1.

What is a typical contour interval on USGS topo maps?

USGS 1:24,000 (7.5-minute) maps most often use 10, 20 or 40 ft, with 5 ft or less in flat areas and 80 ft or more in steep mountains. USGS 1:100,000 maps use 5, 10, 20 or 50 m. The interval is chosen for the terrain and is printed in each map margin.

How do I calculate slope from contour lines?

Rise = contour interval × intervals crossed; run = map distance × scale denominator. At 1:24,000, crossing 5 intervals of 20 ft over 1 inch of map is 100 ft of rise over 2,000 ft of run: a 5% grade or 2.9°.

What does close contour spacing mean?

Lines that are close together mean a steep slope; widely spaced lines mean gentle terrain. The interval itself does not change across a map, only the horizontal distance between lines.

Contour Interval Calculator

Contour interval = elevation difference between two labeled (index) contours ÷ number of spaces between them. With 4 unlabeled lines between the 500 ft and 600 ft index contours there are 5 spaces, so the interval is 100 ÷ 5 = 20 ft.

Contour Interval Inputs

Higher labeled contour

Lower labeled contour

Unlabeled lines between them

Usually 4 on USGS maps (index contour = every 5th line). Intervals = lines between + 1.

Examples

Result

Enter values and click Calculate to see the result.

Slope from Contour Spacing

Rise = interval × intervals crossed. Run = map distance × scale denominator. Updates as you type.

Contour interval

Elevation unit

Intervals crossed

Map distance

Scale 1 :

Typical Contour Intervals by Map Scale

The interval is chosen for the terrain, so always read the value printed in the map margin. Ground distances are computed from the scale.

Scale	Typical series	1 in on map	1 cm on map	Typical contour interval
1:24,000	USGS 7.5-minute quadrangle / US Topo	2,000 ft	240 m	10, 20 or 40 ft (5 ft flat, 80 ft+ mountains)
1:25,000	UK OS Explorer, many national 1:25k maps	2,083 ft	250 m	5 m (10 m in mountains)
1:50,000	UK OS Landranger, Canada NTS	4,167 ft	500 m	10 m (NTS also 20 m)
1:100,000	USGS 30 × 60-minute metric	8,333 ft (1.58 mi)	1.0 km	5, 10, 20 or 50 m
1:250,000	USGS 1 × 2-degree / Alaska	20,833 ft (3.95 mi)	2.5 km	50, 100, 200 or 500 ft (Alaska series)

Sources: USGS Topographic Map Symbols (index, intermediate and supplementary contours; 10 ft or less in flat areas, 100 ft or more in mountains) and USGS map series (1:100,000 at 5, 10, 20 or 50 m; Alaska 1:250,000 at 50, 100, 200 or 500 ft).

Summary

The Contour Interval Calculator finds the vertical spacing between adjacent contour lines on a topographic map. Use two labeled index contours and the number of lines between them, or a total elevation change and the number of intervals. It also works in reverse to count lines for a known interval, and turns contour spacing into slope percent, angle and ground distance for any map scale. A reference table lists typical contour intervals for common map scales.

How it works

Find two labeled index contours (the thicker brown lines with elevations printed on them).
Subtract the lower elevation from the higher one.
Count the unlabeled lines between them and add 1 to get the number of spaces (intervals).
Contour interval = elevation difference ÷ number of intervals. On USGS maps four lines usually sit between index contours, so the interval is the index difference ÷ 5.
For slope, multiply the interval by the number of intervals crossed to get the rise, measure the map distance and multiply by the scale to get the run, then gradient = rise ÷ run.

Use cases

Answering "what is the contour interval of this map" in a geography or geology lab.
Determining the contour interval of a printed topo map when the legend is missing or unclear.
Estimating trail grade or hillside slope from contour spacing before a hike.
Checking GIS-generated contour output against a known elevation range and line count.
Converting map distance to ground distance at 1:24,000, 1:25,000, 1:50,000 or 1:100,000.
Teaching the relationship between contour spacing, interval and terrain steepness.

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu