Quarterly estimated tax calculator
What to pay in instalments during the year, based on the safe-harbour rule rather than a forecast.
Pay each quarter
Estimate$4,500.00
18,000 across four instalments to reach the safe harbour
- Safe harbour target
- $18,000
- Self-employment tax
- $13,423
- Projected tax for the year
- $24,017
| Net profit | $95,000 |
|---|---|
| Self-employment tax | $13,423 |
| Federal income tax | $10,593 |
| 90% of this year | $21,615 |
| 100% of last year | $18,000 |
| Still to pay this year | $18,000 |
- Last year's tax is the cheaper safe harbour here, and it is a known number — you cannot be penalised for paying it even if this year turns out much bigger.
- Withholding from a job counts as paid evenly across the year, so increasing it late can fix an underpayment that quarterly instalments cannot.
Paying 100% of last year’s $18,000 tax protects you from a penalty even if this year is much larger. The safe harbour buys certainty, not accuracy.
Over the life of it
How this is calculated
Safe harbours remove the guesswork
Most systems let you pay a proportion of the prior year’s tax instead of forecasting the current one. In the US that is 100% of last year, or 110% above $150,000 of prior-year AGI.
required = min(90% × this year, 100% or 110% × last year)A known number beats a forecast
The prior-year figure comes from a filed return — $18,000 is $18,000. A current-year estimate can be breached retroactively by income arriving in the final quarter, exposing all 4 earlier instalments to penalty.
Timing of payment matters
Instalments are credited when paid, so a shortfall at the 1st due date keeps accruing even if the 4th overpays. Withholding is usually treated as spread evenly across all 12 months regardless of when it happened.
Worked example: $95,000 of profit with $18,000 of tax last year
| Net profit | $95,000 |
|---|---|
| Last year's tax | $18,000 |
| Target | $18,000 |
|---|---|
| Per instalment | $4,500 |
Instalment rules and safe harbours differ by country. Pick a country above for the deadlines and thresholds that apply to you.
What this assumes
- National tax only, excluding regional instalments.
- Income spread evenly across the year.
- Standard deductions with no credits.
- Sole trader rather than a company.
Where this commonly goes wrong
- Instalment dates are frequently uneven despite being called quarterly, so equal spacing is a false assumption.
- A first profitable year usually has no useful prior-year figure, leaving the current-year forecast as the only protection.
- Regional taxes normally run their own instalment schedule with separate penalties.
Questions
How much should I pay in instalments?
Enough to meet the safe harbour — in the US, 90% of this year’s tax or 100% of last year’s, whichever is lower. On $18,000 of prior-year tax that is $4,500 per instalment.
Why use the prior year rather than a forecast?
Because it is a filed, known number. A current-year forecast can be breached by income arriving late in the year, which exposes every earlier instalment to penalty retroactively.
Does it matter when I pay?
Yes. Instalments are credited on the date paid, so an early shortfall keeps accruing even if a later payment overshoots. Withholding is usually treated as spread evenly instead.
Related tools
Sources
Estimate only, based on published IRS figures for the current period. Not tax or legal advice. Confirm your position with the IRS, a CPA, or an enrolled agent.
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