Centre line method vs long wall–short wall method

There are two standard ways to get wall quantities off a floor plan, and on a simple rectangular building they give exactly the same answer. On anything more complicated they do not, and the difference is not a rounding error.

Knowing which method applies where is basic estimating competence, and it is the thing most commonly got wrong by people who learned one method and apply it everywhere.

The problem both methods solve

A wall running around a building has corners. At each corner, two walls meet, and if you measure each wall to its full outer extent, you count the corner twice. Measure each to its inner extent and you miss it.

Both methods are ways of handling corners consistently. They differ in how.

Centre line method

Measure the total length of the wall centreline around the building, then multiply by wall thickness and height.

Quantity = centreline length × thickness × height

For a rectangular building 10m × 6m externally with 230mm walls:

Centreline dimensions are the external dimensions less one wall thickness in each direction: 10 − 0.23 = 9.77m and 6 − 0.23 = 5.77m.

Centreline perimeter = 2 × (9.77 + 5.77) = 31.08m
Volume = 31.08 × 0.23 × 3.0 = 21.45 cum

The corners handle themselves — the centreline runs continuously through each one, counting it exactly once.

Where it gets adjusted. At every junction where an internal wall meets an external wall (a T-junction) or where two internal walls cross, the centreline needs correcting: half a wall thickness is deducted per junction, because the centrelines overlap there.

When to use it: buildings where all walls are the same thickness. It is faster and less error-prone on regular plans.

Long wall–short wall method

Measure the long walls to their full outer length, and the short walls to their inner length between them.

Long wall length = centre-to-centre length + one thickness
Short wall length = centre-to-centre length − one thickness

Same building:

Long walls: 9.77 + 0.23 = 10.00m each, two of them = 20.00m
Short walls: 5.77 − 0.23 = 5.54m each, two of them = 11.08m
Total = 31.08m
Volume = 31.08 × 0.23 × 3.0 = 21.45 cum

Identical, as expected. The long wall picks up the corner, the short wall gives it back.

When to use it: buildings with walls of different thicknesses. Because each wall is measured separately, a 230mm external wall and a 115mm internal partition are handled independently without any centreline correction.

Where they diverge

The methods agree when every wall is the same thickness. They stop agreeing the moment thicknesses vary, and the centre line method is the one that breaks.

Consider a building with 230mm external walls and a 115mm internal partition. The centreline of the partition and the centreline of the external wall meet at a T-junction — but they are centrelines of walls with different thicknesses, so the correction at that junction is not simply half a thickness. Applying the standard correction gives a wrong answer.

Long wall–short wall handles this without any special case, because it never treats the two walls as one continuous line.

Practical rule: uniform thickness, use centre line for speed. Mixed thicknesses, use long wall–short wall. If you are unsure, long wall–short wall is always correct and only slower.

Worked example with an internal wall

10m × 6m external, 230mm walls, 3.0m height, with one 115mm internal partition running the 6m direction, positioned 4m from one end.

Long wall–short wall.

External long walls: 2 × 10.00 × 0.23 × 3.0 = 13.80 cum
External short walls: 2 × 5.54 × 0.23 × 3.0 = 7.65 cum
Internal partition: clear length between internal faces of long walls = 6 − 0.23 − 0.23 = 5.54m
Partition volume: 5.54 × 0.115 × 3.0 = 1.91 cum

Total: 23.36 cum before opening deductions.

The partition is measured to its clear length between the walls it abuts. Measuring it to the building's full 6m width would double-count the two junction zones already billed in the external walls.

Two errors this method-choice causes

Mixing methods within one estimate. Measuring external walls by centre line and internal by long wall–short wall produces overlaps or gaps at the junctions between them. Pick one and apply it to the whole plan.

Forgetting junction corrections in the centre line method. On a building with several internal walls, the uncorrected centreline overstates masonry by roughly half a wall thickness × height per junction. Six junctions on a 3m-high wall at 230mm is about 0.6 cum — small in isolation, systematic across a project.

What this method-choice does not solve

Both methods give gross wall volume. Everything else still applies: opening deductions, beam-zone deductions, and the separation of external from internal walls for rating. See deductions in brickwork and plaster measurement for those rules.

Neither method touches the outside-to-outside dimension convention used on Indian architectural plans, where plan dimensions are commonly given to the outer face of the wall rather than to centrelines. That convention is what determines your starting numbers, and misreading it shifts everything downstream.

For the full measurement sequence, see how to make a BOQ from architectural drawings. For the units each measured item is billed in, see units of measurement in an Indian construction BOQ.

Frequently asked questions

What is the centre line method of estimation?

Measuring the total length of the wall centreline around a building and multiplying by thickness and height. Corners are counted correctly because the centreline passes through each one once. It requires a deduction at every internal wall junction.

When should I use long wall–short wall instead?

Whenever wall thicknesses vary within the building. The centre line method needs junction corrections that are not straightforward when the meeting walls differ in thickness, whereas long wall–short wall measures each wall independently and needs no correction.

Do the two methods give the same answer?

On a building with uniform wall thickness, yes — exactly. They diverge where thicknesses vary, and in that case long wall–short wall is the reliable one.

How do you measure an internal partition wall?

To its clear length between the internal faces of the walls it abuts, not to the full building dimension. Measuring the full dimension double-counts the junction zones already included in the external wall measurement.

What deduction is needed at wall junctions in the centre line method?

Half a wall thickness per junction, because the centrelines of the meeting walls overlap over that distance. On a plan with several internal walls these corrections accumulate and are commonly omitted.