Map Scale Calculator
Turn a distance measured on a map into a ground distance, find the map length of a known distance, or work out an unknown scale.
What a representative fraction means
A map scale written as 1:24,000 is a representative fraction: one unit on the map equals 24,000 of the same unit on the ground. It works in any unit, which is its advantage over verbal scales such as “one inch to the mile”. At 1:24,000, the scale of the classic USGS 7.5-minute topographic quadrangle, 1 inch on the map is 24,000 inches, exactly 2,000 ft, on the ground. At 1:25,000, the standard for Ordnance Survey Explorer maps and many European hiking maps, 4 cm is exactly 1 km. At 1:50,000, the scale of OS Landranger and many military maps, 2 cm is 1 km.
A worked example
On a 1:24,000 quadrangle, a straight line between two trail junctions measures 4.5 inches. The ground distance is 4.5 × 24,000 = 108,000 inches, or 9,000 ft, 2,743 m, or 1.70 miles. Measuring the same route along the trail with a piece of string might give 6.2 inches, 2.35 miles, because trails wind. Going the other way, a 1 km grid square on a 1:24,000 map is 4.17 cm across, and a mile is 2.64 inches. To find an unknown scale, measure a known ground distance on the map: if a 1 km stretch measures 4 cm, the scale is 1:25,000.
Large scale and small scale
Map makers call 1:24,000 a large-scale map and 1:1,000,000 a small-scale map, which confuses many people. The terms refer to the size of the fraction: 1/24,000 is a larger number than 1/1,000,000, so features appear larger and more detailed. A large-scale map covers a small area in detail; a small-scale map covers a large area with less detail. Hikers usually want 1:24,000 to 1:50,000; road atlases are around 1:250,000 to 1:1,000,000.
Where scale stops being exact
A printed scale is exact only along certain lines of the projection. On a UTM-based topographic map the scale error is less than 0.1% across the sheet, far less than paper shrinking in the rain. On web maps using Web Mercator, the scale changes with latitude by a factor of 1 ÷ cos(latitude): at 60° N, features are drawn twice as large as at the equator, which is why the scale bar on a web map changes as you pan north. A common mistake is to measure from a photocopy or screenshot that has been resized; use the scale bar printed on the same sheet, not the fraction in the margin, when the map may have been enlarged or reduced.
How to use it
- Choose whether you want a ground distance, a map distance or the scale itself.
- Enter the scale denominator, for example 24000 or 25000.
- Enter the map distance in centimetres, millimetres or inches, or the ground distance in your preferred unit.
- Read the converted distance, remembering that straight lines are shorter than the route you will walk.
Questions from the map table
What does a map scale of 1:24,000 mean?
One unit on the map equals 24,000 of the same units on the ground, so 1 inch represents 24,000 inches, which is exactly 2,000 ft, and 1 cm represents 240 m. It is the scale of the USGS 7.5-minute topographic quadrangles that cover the United States.
How do I convert map distance to real distance?
Multiply the map distance by the scale denominator and convert the units. At 1:24,000, 4.5 inches on the map is 4.5 × 24,000 = 108,000 inches, or 9,000 ft, about 1.70 miles. At 1:25,000, each centimetre is 250 m and 4 cm is 1 km.
Is 1:25,000 a large or small scale?
It is a large scale. The larger the fraction, the larger features appear, so 1:25,000 shows more detail over a smaller area than 1:250,000. Hiking maps are usually 1:25,000 or 1:50,000, while road maps are often 1:250,000 or smaller.
Why does the scale bar on a web map change as I scroll?
Most web maps use the Web Mercator projection, which enlarges areas away from the equator by 1 ÷ cos(latitude). At 60° N, things are drawn at twice the size of the same features at the equator, so the scale bar must shrink to stay correct for the area you are viewing.
Disclaimer: Map results are calculations on the WGS84 datum from the coordinates and measurements you enter. They are not a navigation device, a land survey or a safety assessment. Carry a map and compass outdoors, check the datum of your map, apply the current magnetic declination, and rely on deeds and licensed surveyors for property boundaries and areas.