Floor Area Calculator
A developer quotes the gross area. This page subtracts the shared space and the internal walls so you can see the area inside the front door.
The big number is the area left after the shared space is removed. The carpet area is a further estimate after internal walls; set the wall deduction to 0 if you only want the shared-area subtraction. Neither figure replaces a measured survey.
The three areas behind one price
| Name | Chinese term | What it includes |
|---|---|---|
| Gross floor area | 建筑面积 | Everything the building allocates to the unit: interior floor, internal walls, balcony share and a share of lobbies, stairs, lifts and plant rooms |
| Net floor area | 套内建筑面积 | The unit's own boundary: usable floor plus internal walls plus the balcony's counted share |
| Carpet area | 套内使用面积 | The floor you can actually walk on, after internal walls and fixed partitions |
The gap between gross and net is the shared area, 公摊 in Chinese property documents. The gap between net and carpet is the internal wall structure. A developer's headline price is usually per square metre of gross area, while the space you furnish is closer to the carpet area, which is why the difference matters.
A worked example
Take a flat quoted at 100 m² gross with a 20% shared-area ratio and a 5% wall allowance:
- Shared area: 100 × 20% = 20 m²
- Net area: 100 − 20 = 80 m²
- Internal walls: 80 × 5% = 4 m²
- Carpet area: 80 − 4 = 76 m²
The usable ratio is 80%, and the carpet ratio is 76%. If the price was 30,000 per gross square metre, the flat costs 3,000,000; per carpet square metre it is about 39,474. Both statements are true, and the second is the one that describes the room you stand in.
Typical shared-area ratios
| Building type | Shared area | Usable ratio |
|---|---|---|
| Walk-up, six floors or fewer | 7–12% | 88–93% |
| Mid-rise with one lift | 12–18% | 82–88% |
| High-rise with two lifts and a lobby | 18–25% | 75–82% |
| Tower with a podium, clubhouse or wide corridors | 25–30% | 70–75% |
These are rules of thumb, not limits. A building with a large lobby, a refuge floor or a generous stair core can exceed them. The number that matters is the one written in the contract, next to the definition it uses.
How the page calculates
net area = gross area × (1 − shared area % ÷ 100) carpet area = net area × (1 − wall deduction % ÷ 100) usable ratio = net area ÷ gross area × 100Square feet and square metres are converted with 1 m² = 10.7639104167 ft². The calculation is unit-independent: the percentage is applied before the conversion, so switching the unit changes the labels, not the ratio.
What the calculator cannot know
- Which definition the contract uses. Some contracts quote gross area, some net, and some quote a net figure that already includes a balcony at half or full area. The calculator applies your percentage; it cannot read the clause.
- The balcony rule. Enclosed balconies are often counted at full area and open ones at half. That changes the net figure without changing the carpet.
- Wall thickness. The 5% default is an allowance, not a measurement. A flat with many internal partitions loses more; an open plan loses less.
- Shared-area allocation. The ratio is usually building-wide, not per flat. Two flats of the same gross area can carry different shares if the allocation formula is not proportional.
- Measurement error. A plan dimension and a laser measure rarely agree to the centimetre. Treat a difference under 1% as noise.
Check the contract, not the brochure. A marketing floor plan is not a survey. The legally binding figure is the one in the sale contract, and it should state both the gross area and the shared area, plus the rule used for balconies and common space. If only one number appears, ask which definition it uses before comparing two flats.
Questions
Is a 20% shared area normal?
For a high-rise with lifts and an enclosed lobby, 18–25% is common. For a six-storey walk-up, 20% would be unusually high. The ratio is not a quality score: a well-designed building can have a large share because it has generous stairs and a lift lobby, while a cheap building can hide a small share by leaving the lobby unheated and the stairs narrow.
Is the usable ratio the same as 1 minus the shared-area ratio?
Only when the wall deduction is zero. With a 20% share and a 5% wall allowance, the net ratio is 80% and the carpet ratio is 76%. Chinese marketing often calls the net ratio 得房率 (the rate of area obtained), and that figure already excludes the shared space but still includes internal walls.
Can I enter square feet?
Yes. Choose square feet in the unit field and enter the gross figure in square feet; the result and every metric are shown in square feet, with the square-metre equivalent in the last row.
Why does the result change when I change the wall deduction?
Because the big number is the net area after shared space, and the wall deduction only affects the carpet area below it. If you want the headline number to include walls, enter the combined percentage in the shared-area field instead, or look at the carpet-area row.
Does the page store the area I enter?
No. The figures are read by the script, used for the arithmetic and written back into the page. Nothing is uploaded or stored, and reloading restores the 100 m² example.