Contour Interval Calculator
The contour interval calculator helps you work with topographic maps and elevation data. Enter two elevations and a contour interval to find how many contour lines exist between them, or enter a lower elevation, a contour interval, and a count to find the upper elevation. You can also compute the slope percent between two points given the horizontal distance. Contour intervals are the standard unit of topographic map reading used by USGS, civil engineers, foresters, and outdoor planners.
Contour interval formulas
Elevation difference = Upper elevation - Lower elevation
Contour lines = floor(Elevation difference / Contour interval)
Slope (%) = (Elevation difference / Horizontal distance) * 100
USGS standard contour intervals: 10 ft (1:24,000 flat terrain), 20 ft, 40 ft, 80 ft (mountainous). Metric maps: 5 m, 10 m, 20 m.
Reading contour maps
- Closer contour lines indicate a steeper slope; widely spaced lines indicate a gentle slope.
- Contour lines that form a V shape pointing uphill indicate a valley or stream channel.
- Contour lines that form a V shape pointing downhill indicate a ridge or spur.
- Closed contour lines with hachure marks (inward ticks) indicate a depression.
- Every fifth contour line is an index contour, drawn heavier and labeled with the elevation.
Contour interval calculator: frequently asked questions
What is a contour interval?
A contour interval is the difference in elevation between adjacent contour lines on a topographic map. Standard USGS 1:24,000 quadrangle maps use a 40-foot contour interval. Smaller-scale maps may use 10 or 20 feet; larger-scale engineering maps may use 1 or 2 feet.
How do I calculate the slope from contour lines?
Slope = (number of contour lines crossed * contour interval) / (horizontal map distance * map scale). Expressed as a percentage: slope % = elevation difference / horizontal distance * 100.
What is an index contour?
Index contours are every fifth contour line, drawn heavier and usually labeled with the elevation. They help readers quickly identify elevations without counting every line. The intermediate contours between index lines are called intermediate contours.
How many contour lines between two elevations?
Count = (upper elevation - lower elevation) / contour interval. Round to the nearest whole number. If the elevations do not fall exactly on contour lines, the count will depend on where within the interval each elevation sits.
What contour interval should I use for site engineering?
Engineering site plans typically use 1-foot or 2-foot contour intervals for detailed grading work. USGS topographic maps use 10, 20, 40, or 80 feet depending on the map scale and terrain ruggedness. The choice depends on accuracy needed and terrain relief.
Official sources
- USGS: USGS Topographic Maps program.
- NOAA National Geodetic Survey: NGS elevation and vertical datums.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.