The double-subtraction trap: don't count your contributions twice
Your self-employed CPP and QPP contributions are already inside total payable. Subtracting them again costs thousands of dollars, in every province.
Updated
This is the most expensive mistake in a take-home calculation, and it is completely reasonable — which is exactly why it is so common.
The reasoning that leads you into it
You are self-employed. You know you pay CPP or QPP, you pay QPIP, you pay the prescription drug premium. That is real money leaving your account. So it seems obvious to subtract it from your income.
Except it has already been subtracted.
What “total payable” lines actually contain
TP-1 line 450 (Québec) contains:
| Line | What it is |
|---|---|
| 445 | QPP contributions |
| 439 | QPIP contributions |
| 446 | Health services fund contribution |
| 447 | Prescription drug insurance premium |
T1 line 43500 (all of Canada) contains:
| Line | What it is |
|---|---|
| 42000 | Net federal tax |
| 42100 | CPP payable on self-employment |
| 42120 | EI premiums payable on self-employment |
| 42200 | Social benefits repayment |
| 42800 | Provincial or territorial tax |
Both lines are called “total payable” precisely because they total everything you owe.
What it costs
A Québec self-employed person with $75,000 of net business income:
| Total income (15000) | $75,000.00 |
| Total payable (43500) | − $8,200.00 |
| Total Québec payable (450) | − $19,887.90 |
| Québec abatement (44000) | + $1,353.00 |
| Take-home | $48,265.10 |
That $19,887.90 on line 450 breaks down as:
| Québec income tax | $9,500.00 |
| QPP (line 445) | $8,974.40 |
| QPIP (line 439) | $658.50 |
| Drug insurance (line 447) | $755.00 |
Subtracting the QPP, QPIP and drug premium a second time gives $37,877.20 — $10,387.90 poorer than reality.
For a couple, that is more than enough to invert who should be paying what.
This is not a Québec problem
The same mistake exists in every province, on a different line.
A self-employed Ontarian with $70,000 of net business income:
| Net federal tax (42000) | $7,400.00 |
| CPP on self-employment (42100) | $7,913.50 |
| Ontario tax (42800) | $4,200.00 |
| Total payable (43500) | $19,513.50 |
Their CPP is right there on line 42100. Take-home: $70,000 − $19,513.50 = $50,486.50. Subtracting CPP again would strip out $7,913.50 that was never theirs to lose twice.
So why are T4 boxes subtracted?
Because an employee’s deductions are in neither of those lines.
They come off your pay throughout the year and appear nowhere in line 43500 or line 450. That is why boxes 17, 17A, 18, 55, 20 and 44 do get subtracted — and why nothing on the self-employment side does.
The rule
Anything already inside a “total payable” line is never subtracted again.
That single rule covers the whole country and every situation. It also handles the person with both a T4 and a business: their T4 boxes come off, their self-employment side does not, and there is no special case to write.
Our calculator makes this mistake structurally impossible — self-employment data simply has no field that could reach the subtraction. Try it →