Measurement and Unit Guide

Before you start

• Work in one unit per measurement and let the calculator convert, not your head.
• Measure twice and write the figures down before you type them in.
• Measure what exists, not what the drawing says should exist.
• Where a dimension varies, split the area into sections rather than averaging it.

Rectangular areas

Measure the length along the longest side and the width across it, both at the finished edges of the pour rather than the edges of the excavation. If opposite sides differ by more than a few centimetres the area is not truly rectangular; take the larger figure if you want to be safe, or split it into two sections if the difference is significant.

Circular areas

Circles are measured across the widest point, through the centre — that is the diameter, and it is what the calculators ask for. Measuring the radius and doubling it is less reliable, because finding the exact centre on the ground is harder than finding the widest span. Take two diameters at right angles; if they differ, the shape is an oval and you should treat it as two sections or use the larger figure.

Irregular shapes: divide into sections

An L-shaped patio, a drive that widens at the gate or a path that wraps a corner is not one shape — it is two or three simple ones. Draw the outline, cut it into rectangles and part circles with straight lines, measure each piece separately and add a section for each. The calculators are built around this: sections exist precisely so you never have to guess an average.

The one rule when dividing is not to double-count the overlap. Where two rectangles meet at a corner, the corner belongs to one of them only.

Slab thickness

Thickness is measured from the top of the prepared, compacted base to the finished surface level — not from the bottom of the excavation, which usually includes a sub-base. Check it in several places: a dip in the base adds concrete you did not plan for. Run a string line or straight edge across the formwork and measure down from it at intervals.

Trenches and strip footings

Measure the trench along its centre line, not the outside edge, otherwise every corner is counted twice. Width is measured at the bottom where the concrete sits, and depth is the depth of concrete, which is often less than the depth you dug. On sloping ground a trench is usually stepped; measure each step as its own section.

Columns and piers

A column is measured by its diameter (or its two sides, if square) and its height from the top of the footing to the finished top of the pour. If several columns are identical, measure one carefully and set the quantity rather than entering each one.

Post holes

Measure the hole after digging, because augers and spades rarely produce the size you intended. Depth runs from finished ground level to the bottom of the hole. Then measure the post itself across the face, and how far it will sit down into the hole — which is not the hole depth if the post stands on a pad of concrete or hardcore.

Sloped ground

On a slope the depth of concrete changes from one end to the other, and a single average depth quietly hides that. Two better approaches: step the pour and treat each step as a section, or measure the depth at both ends and split the area into several sections with increasing depth. Either way you end up with numbers you can check on site, rather than one figure that is right nowhere.

Remember that horizontal length is what matters for area. Measuring along the slope overstates the plan dimension.

Units: a note on mixing them

It is perfectly fine to enter a length in metres and a depth in millimetres, or a length in feet and a depth in inches — each field has its own unit selector and everything is converted to the same internal base before any maths happens. What you must not do is type a figure in one unit while the selector shows another. If a result looks wildly wrong, that is almost always why.