About
Two percentages, one subtraction and an honest list of what the arithmetic cannot see.
What the page computes
Four inputs, three outputs. The shared-area percentage is applied to the gross area, then the wall allowance is applied to the result. The page shows the intermediate numbers so the subtraction can be checked by hand.
| Step | Formula | Worked example (100 m², 20%, 5%) |
|---|---|---|
| Shared area | gross × share% ÷ 100 | 20 m² |
| Net area | gross − shared area | 80 m² |
| Wall deduction | net × wall% ÷ 100 | 4 m² |
| Carpet area | net − wall deduction | 76 m² |
| Usable ratio | net ÷ gross × 100 | 80% |
Input handling
- Gross area must be greater than zero. There is no upper limit in the arithmetic; the field suggests 100,000 to catch a misplaced decimal point.
- Shared-area percentage is clamped to 0–100. A negative value is treated as 0, and a value above 100 as 100, which produces a net area of zero rather than a negative one.
- Wall allowance is clamped to 0–100 for the same reason. The default of 5% is an allowance for internal partitions, not a measurement.
- Unit switches the labels and the conversion, not the percentages. The ratio is identical in square feet and square metres.
- Recalculation happens on every keystroke and every select change; there is no submit button.
Unit conversion
One square metre is 10.7639104167 square feet. The page converts the result, not the input: the percentage is applied in the selected unit, which is mathematically the same as converting first and rounding second for a linear conversion. The last metric row always shows the net area in the other unit, so a mixed-unit comparison does not need a second visit.
Where the definitions come from
The three-level description follows the Chinese property measurement framework: gross floor area (建筑面积), net floor area (套内建筑面积) and usable or carpet area (套内使用面积). The national measurement standard is GB/T 17986, and the allocation of common area in commercial housing follows the rules on sales area and common-area allocation. Those documents define what may be counted; they do not fix one percentage for every building.
The page does not implement the standard's balcony coefficients or wall rules. It applies the percentage you enter, which is the part a buyer can check against a contract. A legal measurement is a surveyor's job, not a web page's.
What the calculator does not know
- Which definition the price uses. The page cannot tell whether the quoted area is gross, net or carpet. It only computes what the labels say.
- The balcony coefficient. Half-area and full-area counting change the net figure. They are not modelled separately.
- The actual wall layout. A 5% allowance is an average. An open-plan flat has less wall; a flat with a long corridor has more.
- Shared-area allocation disputes. Whether a clubhouse or a car park should be in the residential shared area is a contract and planning question.
- Measurement error. Plan dimensions and site measurements differ. A small difference is normal.
How the pages are built
Four static pages, one stylesheet and one script, served from this domain. No framework, no third-party script, no analytics, no cookie. The arithmetic is a single function with no dependencies, and the worked example above is the one used to check a change.
Corrections
If a formula or a boundary is wrong, the fix belongs in the script and this page. The tests cover zero and hundred-percent inputs, the unit switch and the worked example, so the calculator and the description cannot drift apart silently.