Working out built up area from carpet, step by step

By Parish Rao · 17 September 2026 · 7 minute read

Most explanations of built up area stop at the definitions. That is the easy half. This one takes a single flat and does the arithmetic in both directions, because the direction you need depends on which number somebody has handed you.

The flat we are working with

A two bedroom flat. The agreement states a carpet area of 700 square feet. The builder advertised it as 1,000 square feet. Both numbers are on paper and neither is a lie.

Everything below uses that one flat, so you can follow a single set of numbers rather than a general rule.

Going up: carpet to built up

Built up area is carpet area plus the walls. The walls are not free space, but they are part of the structure you paid to build.

The allowance is normally 10 to 15 percent. Take 10 percent on our flat:

700 + (700 x 10 percent) = 700 + 70 = 770 square feet built up.

Why the range? Thickness and layout. External walls in a coastal building are thicker. A flat chopped into four small rooms has more internal partitions than a flat with two large ones. Same carpet area, different wall load.

Going up again: built up to super built up

Super built up adds your share of everything the building owns in common. Lobby, staircase, lift shaft, corridors, and in many projects the clubhouse, gym and society office.

That share is called loading, and it is expressed as a percentage of built up area. On our flat the builder loads 30 percent:

770 + (770 x 30 percent) = 770 + 231 = 1,001 square feet super built up.

There is the builder's thousand. The flat did not grow. The measurement changed twice.

Going backwards: builder quote to carpet

This is the direction most people actually need, because the advertisement is where they start. Reverse the two steps, dividing instead of multiplying.

Step one, strip the loading: 1,000 divided by 1.30 = 769 square feet built up.

Step two, strip the walls: 769 divided by 1.10 = 699 square feet carpet.

Notice that you divide rather than subtract. Taking 30 percent off 1,000 gives 700, which is wrong, because the 30 percent was calculated on the smaller number. That single mistake is the most common error people make here, and it flatters the flat by about 40 square feet.

What loading does to the price

Say the quoted rate is Rs 9,000 per square foot on super built up area.

Total price: 1,000 x 9,000 = Rs 90,00,000.

The money buys 700 square feet of floor you can stand on. So the rate on space you can actually use is:

90,00,000 divided by 700 = Rs 12,857 per square foot of carpet.

That is the number to carry between two projects. A tower quoting Rs 8,500 with 40 percent loading is more expensive per usable foot than one quoting Rs 9,000 with 20 percent loading, and the advertisement will never tell you that.

Loading is not capped

No law limits it. Two buildings on the same road can load 20 percent and 40 percent. The second gives you meaningfully less flat for the same money, and it is entirely legal, because the loading was disclosed.

Under RERA a builder must quote and sell on carpet area, and carpet area now has a statutory definition. Super built up survives in conversation, in older resale listings and in how people describe their own homes, which is why the confusion has outlived the rule that was meant to end it.

Which number goes in a valuation report

None of the builder's. A valuer measures the flat and reports what was found on the ground, stating clearly which area is being reported.

This surprises owners at bank valuations. The report says 700 and the owner has believed 1,000 for a decade. Nothing is wrong. The bank wants the area that exists, not the area that was marketed.

You can run these numbers for your own flat in the carpet area calculator, which does both directions and shows the real rate per usable foot. In Maharashtra, where pre 2017 agreements used a different definition again, use the MOFA to RERA converter instead.