BuildMetricLab

Surveying & Mapping

Contour Interval Calculator

Calculates contour interval and slope percentage from map data

Updated 26 June 2026 · Live

What this tool does

Calculates contour interval and slope percentage from map data.

Contour Interval Calculator — for a map scale of 1:{scale_denominator} and average gradient of {gradient_pct}%, the recommended contour interval is {result}.

Inputs
%
Result

Recommended Contour Interval

2 m

Map Scale
1:1000
Avg Gradient
5.0%
Raw Calculation
2.00 m (rounded up to standard)
Typical OS Data
1 m on flat terrain, 5 m on upland / hill

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Formula Used
Recommended contour interval
Map scale denominator
Average gradient percent
## How the contour interval calculator works This calculator suggests a contour interval (CI) for a map, based on the map scale denominator and the average gradient (slope) of the terrain you enter. Contour interval is the vertical height difference represented by each contour line on a topographic map or plan. Enter the scale denominator (e.g. 1000 for a 1:1000 plan, or 25000 for a 1:25,000 topographic sheet) and an average gradient as a percentage. The tool computes a base interval from the scale, adjusts it for how steep the terrain is, and rounds up to a standard interval value such as 0.5 m, 1 m, 2 m, 5 m or 10 m. Every result is computed in your browser from the numbers you enter — nothing is looked up from a database of real Australian terrain. ### What the inputs mean The map scale denominator (the 'D' in 1:D) controls how much ground each unit of the map represents — larger denominators mean a smaller-scale map showing more ground, which generally suits a wider contour interval. The average gradient percentage represents how steep the terrain is on average; steeper ground pushes the recommended interval wider so contour lines are not so closely packed they become unreadable, while flatter terrain suggests a tighter interval to retain relief detail. ## What this tool does not do This calculator does not survey your site, does not measure actual elevation data, and does not replace a licensed cadastral or engineering surveyor. It gives a planning-stage suggestion for choosing a sensible contour interval when preparing or interpreting a map or plan. It does not check compliance with the Intergovernmental Committee on Surveying and Mapping (ICSM) standards, state survey regulations, or any requirement of the National Construction Code (NCC) — those judgements need a suitably qualified surveying professional reviewing your specific site and drawings. ## On-site considerations for contour interval Real terrain is rarely uniform: gradients vary across a site, and features like cuttings, drainage lines, retaining structures and rock outcrops can all justify a tighter local interval than the site average would suggest. Surveyors commonly tighten the interval near building envelopes, driveways and drainage paths where small changes in level matter for design, even if the broader site gradient supports a wider interval elsewhere. ## The National Construction Code (NCC) and compliance Where contour and level information feeds into stormwater design, site classification, or footing and slab design under the NCC, the levels and contour data used must come from a proper site survey carried out or checked by a licensed surveyor, not from this planning estimate. This tool is for early-stage map preparation only. ## Before you order If you are commissioning a survey or requesting a topographic plan from a surveyor, use this calculator's suggested interval as a discussion starting point, not a specification. Confirm the final interval with your surveyor based on the actual site relief, the intended use of the plan (e.g. subdivision, drainage design, earthworks), and any council or state survey authority requirements that apply. ## Adjusting the defaults Try different scale denominators and gradients to see how the recommended interval changes — for example, a 1:1000 plan on flat ground (0–2% gradient) typically suggests close to a 1 m interval, while the same scale on steep terrain (15%+) will suggest a wider interval such as 5 m. ## Using this contour interval calculator alongside other BuildMetricLab tools Pair this tool with the bearing-distance calculator when plotting traverse lines onto a contour plan, the land area converter when reporting site areas from a contoured survey, and the GPS coordinate converter when cross-referencing plotted contour points against GPS-derived coordinates.

Sources & methodology

Calculates contour interval and slope percentage from map data. Every result is calculated from the values you enter, and all inputs are editable.

Frequently asked questions

Are contour interval calculator results accurate enough for professional mapping use?

Use this result as a starting estimate for choosing a sensible contour interval only. It is not a substitute for professional cartographic judgement or survey-grade analysis — verify the chosen interval against the relevant mapping standard and the specific terrain and purpose of your map before finalising a plan.

How does gradient affect the recommended contour interval?

Steeper average gradients push the recommended interval wider (fewer, more widely spaced contour lines), while flatter terrain suggests a tighter interval so relief detail isn't lost. The calculator adjusts the base interval derived from map scale by the entered gradient percentage, then rounds up to a standard interval value (e.g. 0.5 m, 1 m, 2 m, 5 m, 10 m).

Does this tool replace a licensed surveyor?

No. This is a planning estimator for choosing a sensible contour interval when preparing or interpreting a map. For any survey used in legal boundary determination, engineering design, or submissions requiring compliance with the National Construction Code (NCC), engage a licensed surveyor or suitably qualified professional.

What map scale denominator should I enter?

Enter the denominator of your map's scale ratio — for example, enter 1000 for a 1:1000 plan or 25000 for a 1:25,000 topographic map. Larger denominators (smaller-scale maps) generally suit wider contour intervals.

Spotted something off?

Calculations or display — let us know.

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