Quarterly Estimated Tax: the Safe Harbour Nobody Explains
You do not have to guess this year's tax bill correctly. Pay 100% of what you owed last year and you are protected from penalties whatever happens - $16,000 instead of $21,600 in this example. But if you earned over $150,000 last year that becomes 110%, and missing it is how people get caught.
You do not have to guess right
The thing that makes estimated tax frightening is the feeling that you must predict this year’s income accurately or be punished.
You do not. The IRS gives you two routes and lets you take whichever is smaller:
- 90% of this year’s tax, or
- 100% of last year’s tax
Hit either and you are protected from the underpayment penalty, even if you end up owing far more than you paid.
Say you expect to owe $24,000 this year and owed $16,000 last year. From the estimated tax calculator:
| Route | What it needs |
|---|---|
| 90% of this year | $21,600 |
| 100% of last year | $16,000 |
| Your target | $16,000 - $4,000 a quarter |
$5,600 less, and last year’s figure is a number you already know rather than a forecast.
You still owe the full tax at filing time. The safe harbour protects you from the penalty, not the bill - so set the difference aside.
The 110% rule that catches people
If your income last year was over $150,000, the last-year route is not 100%. It is 110%.
| Amount | |
|---|---|
| Tax owed last year | $40,000 |
| Expected this year | $60,000 |
| 90% of this year | $54,000 |
| 110% of last year | $44,000 |
Pay the $40,000 you owed last year - which sounds exactly like following the rule - and you are $4,000 short and penalised.
The threshold is $75,000 if you are married filing separately.
This is the single most common way people get caught, because the 100% version is the one everybody has heard.
When your income drops, switch routes
The rule works both ways and this half gets ignored.
Owed $30,000 last year, expecting $10,000 this year? Your target is 90% of this year - $9,000 - not $30,000.
Plenty of people keep paying last year’s amount through a bad year and hand over money they did not need to. It comes back at filing, but it is your cash flow in the meantime, and cash flow is exactly what is tight in a quiet year.
Missing a deadline does not reduce the bill
The four payments are due in April, June, September and January.
If you need $16,000 covered and two deadlines have already gone with nothing paid, the remaining two are not $4,000 each. They are $8,000 each.
Miss enough of them and the last payment becomes impossible, which is how a manageable tax bill turns into a crisis. Falling behind early is the expensive mistake.
A simpler alternative
If you or a partner also have a regular job, extra withholding from that pay is treated as paid evenly across the year, regardless of when it was actually taken.
That means you can fix an underpayment in December that quarterly payments could not, because the withholding is backdated in the eyes of the rules.
It is one of the more useful things to know if you have both kinds of income.
What to do
- Find last year’s total tax - the line on your return, not what you paid at filing.
- Check whether you crossed $150,000. If so, it is 110%.
- Take the smaller of the two routes.
- Divide by four and set up the payments.
- Put the shortfall aside separately. The safe harbour stops the penalty, not the bill.
Work out your own target and quarterly amounts in the estimated tax calculator - it checks whether the 110% rule applies to you and shows what to do if you are already behind.
Common questions about estimated tax
What is the safe harbour for estimated tax?
You avoid a penalty if you pay at least 90% of this year's tax or 100% of last year's, whichever is smaller. On $24,000 expected this year and $16,000 owed last year, the target is $16,000 rather than $21,600.
What is the 110% rule?
If your income last year was over $150,000, the last-year route rises from 100% to 110%. On $40,000 of tax last year that is $44,000, not $40,000. Paying the number you owed last year leaves you short and penalised.
What if my income dropped this year?
Use the 90%-of-this-year route instead. The rule lets you take whichever is smaller, so a quieter year means much smaller payments. Many people keep paying last year's figure and hand over money they did not have to.
What happens if I miss a deadline?
The bill does not shrink - the remaining payments have to carry it. If you need $16,000 covered and two deadlines are gone, the last two become $8,000 each instead of $4,000. Missing early is expensive later.
Do I have to pay at all?
Not if you will owe under $1,000 after withholding and credits. Above that, the safe harbour rules apply. Increasing withholding at a day job is an alternative to quarterly payments and counts as paid evenly through the year.