Four deductions come off a regular Ontario paycheque, and every one of them follows a published CRA formula — not a guess, not a lookup table somebody eyeballed. The rulebook is the CRA's T4127, Payroll Deductions Formulas (122nd edition, in force January 1, 2026). Here's the math, with one worker's numbers in it.
Take a glazier earning $2,560.00 gross for a two-week run. CPP is 5.95% of pensionable earnings after the per-period basic exemption is subtracted: 0.0595 × ($2,560.00 − $134.61) = $144.31. EI is simpler — 1.63% of insurable earnings, with no exemption: 0.0163 × $2,560.00 = $41.73. Both have annual maximums, so once a worker hits the year's ceiling, the deduction stops.
Income tax is the part most people can't reproduce, because it isn't a flat percentage. T4127 annualizes the pay, applies the federal and Ontario rate brackets against the worker's claim code, subtracts the basic personal amounts and credits, then divides back down to the pay period. That's how the same $2,560.00 produces $262.28 federal and $113.64 Ontario for claim code 1 — every step is a formula, not a black box.
That's the whole point of Onest: every line on the paystub shows the exact T4127 formula with this worker's numbers substituted in, so you — or your accountant — can check it. You don't have to trust the software. You can read the math.