Contour Interval Volume Estimator

The Contour Interval Volume Estimator computes earthwork cut/fill volume from a series of contour-line areas at a fixed contour interval — the standard "contour method" civil engineers and surveyors use to estimate volume from a topographic map without a full 3D model.

How to use it

1. Enter the contour interval — the fixed vertical distance between successive contours, in whatever unit your survey uses (meters or feet). The result comes out in that same unit, cubed.
2. Enter one planimetric area per line, in the order the contours appear (lowest elevation to highest, or the reverse — just stay consistent). These are the areas enclosed by each contour, as measured off the map (a planimeter reading, or a GIS polygon area).
3. If one end of your contour series closes to a point — an isolated hilltop or the bottom of a pit, rather than an open-ended excavation boundary — check the matching "closed hilltop or pit" box to apply a cone correction to that one endmost segment.
4. Read the total volume in the summary table, then the per-segment breakdown showing exactly how each pair of contours was combined.

The average end area method

For each pair of adjacent contours, the volume of the slice between them is approximated as a prismatoid — averaging the two areas and multiplying by the contour interval:

volume_segment = ((Area_1 + Area_2) / 2) × contourInterval

The total volume is the sum of every segment between the first and last contour entered. This is the standard method used across civil engineering and surveying for contour-based earthwork takeoffs.

Cone correction for a closed hilltop or pit

Averaging works well between two contours that both represent real cross-sections of a cut/fill boundary. It breaks down at the very top of an isolated hill or the very bottom of an isolated pit, where the innermost contour is really just enclosing a small area around a single high (or low) point — treating that segment as a frustum overstates the volume near the point. Flagging that end applies a cone formula to that one segment instead:

volume_segment = (contourInterval / 3) × A_bottom

where A_bottom is the other area in that segment (the flagged contour is treated as collapsing to a point, contributing no base area of its own — the same way a cone's apex has zero area). Leave both boxes unchecked for the common case: an open cut/fill site with no closed summit or basin at either end.

Is my data private?

Yes. The entire calculation runs locally in your browser — your contour areas never leave your device.

How accurate is this?

The average end area method is an approximation, not an exact integral of the ground surface — accuracy improves as the contour interval gets smaller (more, closer-together contours track the actual terrain more faithfully). For a more precise result across the whole stack of contours, the prismoidal formula (which also accounts for the rate of change of area, not just its endpoints) is more accurate but needs an odd number of contours and more input data; this tool implements only the average-end-area/cone method described above. Treat the output as a solid working estimate for a takeoff, not a certified final quantity.

What if I only have 2 contours?

You'll get one segment. If you flag both ends as a closed hilltop/pit with only 2 areas entered, the flags contradict each other on a single segment (there's no way to have a point at both ends of one frustum), so the tool falls back to plain average end area for that segment rather than double-applying the correction.

Common uses

Estimating cut/fill quantities for a grading or excavation takeoff, checking a contractor's volume estimate against a topographic survey, and quick site-volume sanity checks before committing to a full 3D earthwork model. For land-survey traverse work rather than volume, see the Survey Traverse Closure Calculator and Traverse Adjustment Bowditch tools.