Travel-Time Isochrone Radius Calculator
An isochrone is the boundary of everywhere you can reach within a set travel time. A precise isochrone follows the road network, but a quick circular approximation is useful for retail site selection, service-area sizing and accessibility checks. Straight-line distance is just speed times time, then divided by a detour (circuity) factor to account for roads not being straight. This calculator returns the approximate radius and the circular catchment area in both miles and kilometres. The detour factor is a user input because circuity varies widely by location.
Isochrone radius formula
straight distance = speed * (time / 60)
radius = straight distance / detour factor
catchment area = pi * radius^2
1 mile = 1.609344 km
Time is converted from minutes to hours by dividing by 60. The detour factor reduces the straight-line distance to reflect real road networks. The catchment is the area of the circle of the resulting radius.
Isochrone context
- A circular isochrone is a first-order approximation; real ones follow the road network and are irregular.
- Circuity (network distance over straight-line distance) is typically above 1 in built-up areas.
- A detour factor of 1.0 assumes perfectly straight roads (an upper bound on reach).
- Use the catchment area to size populations or markets within a drive time.
- For exact service areas, use a network routing engine with live or modelled speeds.
Isochrone radius: frequently asked questions
What is an isochrone?
An isochrone is a line connecting all points reachable within the same travel time from an origin. The area inside it is the catchment. A true isochrone follows the road network; this calculator gives the simpler circular approximation: a straight-line radius and circular catchment area.
How is the radius estimated?
Straight-line distance equals speed times time. Because roads are not straight, we divide by a detour (circuity) factor that you set, so radius = (speed times time) / detour factor. A detour factor of about 1.3 is a common rule of thumb for urban road networks, but it varies, so it is a user input.
Why is the detour factor a user input?
Circuity (the ratio of network distance to straight-line distance) depends heavily on the local street grid, geography and barriers like rivers. There is no single universal value, so we never hardcode it. Enter a factor that fits your area; 1.0 means perfectly straight roads.
How accurate is a circular isochrone?
It is a first-order estimate only. Real isochrones are irregular because speeds, intersections and one-way systems differ by direction. Use this to size a rough catchment or sanity-check a routing engine, not as a substitute for network-based isochrone software.
What units does it use?
Enter speed in miles per hour and time in minutes; the radius is reported in miles and kilometres, and the catchment area in square miles and square kilometres. The conversions are exact (1 mile = 1.609344 km).
Official sources
- U.S. Bureau of Transportation Statistics (USDOT): travel time and accessibility data.
- U.S. Geological Survey: geospatial distance and area methods.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.