Two PSF figures, two definitions
Saleable PSF is price ÷ saleable area. Gross PSF is price ÷ gross area. Same flat, same price, different denominator — so the results differ by construction.
Take HK$8M with 500 sq ft saleable and 650 sq ft gross: saleable PSF is HK$16,000, gross PSF about HK$12,308, and the efficiency ratio is 76.9%. The spread is nearly HK$3,700, which is why one sentence can carry two very different levels of persuasion.
Saleable area counts only the floor space inside the unit. Gross area also includes a share of common parts. That is precisely why gross PSF is naturally lower than saleable PSF.
Two flats, one budget: a checking sequence
Step one: convert both flats to the same basis. If one comes with gross area only and the other with saleable area only, convert first — never put A's saleable PSF against B's gross PSF.
Step two: compare efficiency ratios. For the same price and saleable area, the flat with the higher efficiency ratio gives the occupier more genuinely usable space per dollar.
Step three: work out how much the client pays for the non-saleable portion. The tool reports this figure: at HK$8M and a 76.9% efficiency ratio, roughly HK$1.85M of the price is paid for non-saleable area.
Step four: give the client both PSF figures and the efficiency ratio together, not one figure alone. A mixed-basis comparison is exactly the misunderstanding that is hardest to explain later, when the client finds another flat nearby quoted a few thousand dollars cheaper.
Where the efficiency difference comes from
Efficiency depends on building design: the number of floors and units, and the proportion taken by common facilities such as the lobby, lifts, escape stairs and podium. A single-block building, a large estate and buildings from different decades do not share the same efficiency ratio.
That is why even 'same district PSF' needs a consistent basis first. Part of the PSF gap between two flats in one district can come purely from a different efficiency ratio rather than a real difference in price level. This piece provides the method only, and quotes no PSF reference value for any estate.
When only a gross-area figure is available
Some sources provide gross area only. The way to handle it is not to derive a saleable PSF from it, but to label it plainly as a gross-area figure and say so when quoting numbers to the client.
If other units in the same estate do carry saleable area, you can convert once using the efficiency ratio of the same estate and unit type — but that value is an estimate and must be labelled as one. Applying one estate's efficiency ratio to another estate amplifies the error and is not advisable.
One self-check
Enter the price, saleable area and gross area of both flats you want to compare into the saleable vs gross PSF calculator, and read the two figures and the efficiency ratio off the same basis. If one flat is missing its saleable area, go and find it rather than assembling a conclusion out of a gross-area figure.