Two founders can both pay "25% corporate tax" and write wildly different checks. Statutory is the headline in the tax code; effective is what you pay after credits and deductions. Mix them up and your budget meets a stranger. - R&D credits, patent boxes, accelerated depreciation, loss carryforwards: all move the effective rate, sometimes a lot. - A calculator gives you arithmetic; your situation gives you the rate. Pick one with current data. - Incentives phase in and out. A credit generous last year may be gone now. Check the effective date, not just the number. - A corporate tax database by country and year carries rate history, because your forecast depends on whether the incentive survives the budget cycle. Ask for the effective number early. The free table itself is enough to start. And when the numbers need a human, IncorpTrack's UK and Singapore registration runs $49 plus the government fee, banking help included.