Singapore doesn't have a separate tax on rental income. Instead, your net rental income (rent received, minus allowable expenses) simply gets added to your other chargeable income for the year and taxed at the same resident progressive rates that apply to your salary. That means the real tax cost of renting out a property depends heavily on what tax bracket your other income already puts you in.
For non-interest expenses, IRAS gives landlords a choice: claim a flat 15% of gross rental income with no paperwork, or add up your actual property tax, fire insurance, repairs, maintenance, and agent's commission if that total is higher. Mortgage interest sits outside both methods entirely, it's always claimed separately, on top of whichever non-interest method you choose, and only the interest portion of the loan counts, never the principal you're repaying.
Because rental income stacks on top of your existing income rather than being taxed on its own, the marginal cost of a rental property can be higher than its standalone tax rate suggests, especially if it pushes part of your income into a higher bracket. This calculator computes that bracket-crossing effect directly rather than applying a single flat rate.