Gross vs Net Floor Area

Three numbers describe the same flat. Only one of them is the floor you can put a sofa on.

One flat, three areas

Property listings mix three measurements that are not interchangeable. The difference between them can be a whole room's worth of floor, so it is worth separating them before comparing prices.

Measure	What is inside the boundary	Typical use
Gross area (建筑面积)	The unit plus its share of stairs, lifts, lobby, corridors, plant rooms and the external wall's counted half	Sale price, property tax base, developer's statistics
Net area (套内建筑面积)	The unit's own walls, internal partitions and the counted balcony area	Contract comparison, 得房率 marketing
Carpet area (套内使用面积)	The floor inside the internal walls, excluding fixed partitions	Furniture planning, rent per usable square metre

A 100 m² gross flat with a 20% shared area has 80 m² net. A further 5% wall allowance leaves 76 m² of carpet. The same flat can be advertised as 100, 80 or 76 depending on which number the seller wants to lead with.

Why shared area exists at all

A building's stairs, lifts, lobby, machine rooms and structural walls are real construction. Someone paid for them, and every owner uses them. The shared-area ratio is the mechanism that divides that cost between units. The argument is not whether the space exists; it is how much of it each flat is asked to carry, and whether the allocation is transparent.

The allocation is supposed to follow the building's measured common area divided by the sum of each unit's net area. In practice, older rules and local practice differ, and the ratio can be rounded or negotiated. That is why two buildings with the same physical layout can quote different percentages.

The arithmetic of price per usable square metre

If a flat is 100 m² gross at 30,000 per gross square metre, the price is 3,000,000. Divide by the 76 m² carpet area and the effective price is about 39,474 per carpet square metre. The second figure is not the price you pay; it is the price of the floor you can furnish. Comparing two flats on that basis removes the effect of a generous or stingy shared-area ratio.

price per carpet m² = total price ÷ carpet area

The same calculation with a 15% share and the same 5% wall allowance gives 80.75 m² carpet and about 37,152 per carpet square metre — a 6% difference in what the same money buys, from the shared-area clause alone.

What to check in a sale contract

Both figures are stated. A contract that gives only the gross area is incomplete for comparison. Look for the net area and the shared area, not just the total.
The balcony rule. Enclosed balconies are often counted at full area, open balconies at half. The rule changes the net area without changing the room sizes.
The shared-area list. Which spaces are included: the lift lobby, the stair core, the guard room, the clubhouse, the car park? A clubhouse or a car park counted into residential shared area is a common complaint.
The measurement standard. Chinese practice refers to the national property measurement rules (GB/T 17986) and the rules on commercial housing sales area. The contract should name the standard it uses.
The tolerance clause. New-build contracts often allow a small difference between the contracted and delivered area, with a price adjustment above a threshold. Read the threshold before signing.

How to measure your own flat

For a rough check, measure each room's internal dimensions, multiply length by width, and add the results. That gives the carpet area, not the net area: it excludes internal walls and the balcony. Add the wall footprints (a 100 mm partition along a 4 m wall is 0.4 m²) and the counted balcony share to compare with the contract's net figure.

A laser measure is more accurate than a tape for room diagonals, but the limiting factor is usually the definition, not the instrument. If your sum is within about 2% of the contracted carpet area, the difference is probably walls and rounding. A gap of 10% is a question for the developer.

Two mistakes that distort the comparison

Comparing gross with carpet. A 90 m² gross flat and an 85 m² carpet flat are not 5 m² apart; they may be 20 m² apart. Convert both to the same measure before comparing.

Treating the shared-area ratio as a quality score. A low ratio can mean efficient planning, or it can mean a narrow stair core and no lift lobby. A high ratio can mean generous common space, or it can mean a clubhouse you never use. The number describes allocation, not value.

Where the percentage comes from

The shared-area percentage is not a physical constant. It is the result of a measurement and an allocation rule, and it can be revised if the building's common area is re-measured. The calculator takes the percentage as an input rather than pretending to derive it from the building type, because two buildings of the same height can allocate differently.

If you have the building's total common area and the sum of all units' net areas, the ratio is the common area divided by the total gross area. That is the figure to ask for when a sales office quotes a percentage without showing its working.