Estimate Quarterly Tax on Irregular Freelance Income

If you freelance, your income probably doesn't arrive in neat monthly chunks. A big project lands in May, then nothing until August, then two clients pay in the same week. That's normal. What's not so simple is the tax: the Inland Revenue Department still expects four roughly equal quarterly payments across the year, even though your income is anything but even.

So how do you estimate quarterly tax on irregular income in Sri Lanka without either overpaying early or getting caught short later? The answer is to forecast the whole year once, pay against that forecast, and revise the moment your outlook changes. This guide walks through the method with a real LKR example, then shows how to adjust when a quarter surprises you.

Why is quarterly tax harder when your income is irregular?

A salaried employee never thinks about this. Their employer deducts Advance Personal Income Tax (APIT) every month, so their tax tracks their pay automatically. Freelancers don't have that. You're what the Inland Revenue Act calls an "instalment payer," and you settle your own tax in four instalments through the year.

Here's the catch. The instalment system is built around your estimated annual tax, not your actual income in each quarter. The law doesn't ask "what did you earn in the last three months?" It asks "what will you owe for the whole year, and have you paid a fair share of it by now?"

When income is steady, that distinction barely matters. When it's lumpy, it matters a lot. If you size your August payment off a single big project that just paid out, you'll overpay. If you size it off a quiet stretch, you'll underpay and risk a penalty later. The fix is to stop thinking quarter by quarter and start with a full-year estimate.

What method should I use to estimate my quarterly tax?

You file a Statement of Estimated Tax Payable (most people just call it the SET). It's your best forecast of the tax you'll owe for the full year, and you submit it by the first instalment date, August 15.

The SET is not a binding commitment. It's an estimate, and the law expects estimates to move. You file one figure now, pay against it, and if your income picture changes you file a revised one. The Inland Revenue Department cares that your total tax for the year gets paid on a reasonable schedule, not that you predicted your freelance income perfectly in August.

The method has three moving parts:

1. Estimate your annual tax (the SET figure).
2. Divide it across the remaining quarters.
3. Revise when reality diverges from the forecast.

Let's do each one with numbers.

How do I calculate my estimated annual tax? (Step 1)

Take Priya, a freelance UX designer in Colombo. Her clients are overseas, they pay her in US dollars, and the money comes into her Sri Lankan bank account. In April, looking at her signed contracts and her pipeline, she expects to clear about Rs. 6,000,000 in net business income for the year (that's after deducting her allowable business expenses).

First, subtract the personal relief every resident individual gets:

Now apply the progressive slabs for the Year of Assessment 2026/2027:

But Priya's income is foreign service income, and that gets special treatment. Because she provides her service to clients outside Sri Lanka, gets paid in foreign currency, and the money is remitted through a Sri Lankan bank, her tax rate on that income is capped at 15% under the First Schedule, Paragraph 1(6) of the Inland Revenue Act.

So her tax is the lower of the two figures:

• Progressive calculation: Rs. 1,032,000
• 15% cap: 15% of Rs. 4,200,000 = Rs. 630,000

The cap wins. Priya's estimated annual tax is Rs. 630,000.

How do I split the estimate into four quarters? (Step 2)

This part is the easy bit. Take the annual estimate and divide by four.

Rs. 630,000 ÷ 4 = Rs. 157,500 per quarter.

If Priya had any tax already withheld at source, say Advance Income Tax (AIT) on bank interest, she'd subtract that from the annual figure before dividing. For a freelancer earning purely foreign service income with nothing withheld, the plain quarter-split is exactly right.

Notice what this does for irregular income. Priya pays Rs. 157,500 in August whether or not a client happened to pay her that week. The instalment is tied to her full-year forecast, so a quiet July doesn't change it. That's the whole point of estimating annually: it smooths the lumps.

What happens when my income changes mid-year?

This is where freelancers get nervous, and where the system is actually kinder than people expect.

Say it's October. Priya just signed a large retainer she didn't see coming in April. She now expects to clear Rs. 8,000,000 for the year, not Rs. 6,000,000. Her August instalment of Rs. 157,500 is already paid and was correct at the time. What about the rest?

She files a revised SET before the November 15 instalment. Under Section 91(5), an instalment payer can revise the estimate at any time during the year. The revised figure applies to instalments due after she files it (Section 91(6)), so it doesn't claw back the August payment. It just resets the quarters ahead.

The law spreads the new target using a formula. Each remaining instalment equals:

(A − C) ÷ B

• A is the revised annual tax target.
• C is the tax already paid this year.
• B is the number of instalments left, including the one you're calculating.

Let's run Priya's revision. Her new taxable income is Rs. 8,000,000 − Rs. 1,800,000 = Rs. 6,200,000. At the 15% cap, her revised annual tax is 15% of Rs. 6,200,000 = Rs. 930,000.

• A = Rs. 930,000 (revised annual tax)
• C = Rs. 157,500 (the August instalment already paid)
• B = 3 (November, February, May still to come)

Each of the three remaining instalments = (930,000 − 157,500) ÷ 3 = 772,500 ÷ 3 = Rs. 257,500.

Add it up: 157,500 + (257,500 × 3) = Rs. 930,000. Exactly her revised annual estimate. The catch-up from the higher income gets shared evenly across the three quarters that remain, instead of dumped on one. If her income had fallen instead, the same formula would lower the remaining instalments the same way.

Should I use last year's tax or this year's estimate?

There's a wrinkle worth knowing about for the 2026/2027 year and beyond. The Inland Revenue (Amendment) Act, No. 11 of 2026 changed the default basis for instalments.

The new default is simpler: your instalment is based on the tax you paid in the preceding year, divided across the quarters. If you earned the same every year, you wouldn't need to forecast at all. You'd just repeat last year.

But freelancers rarely earn the same every year, and the law accounts for that. If you had no taxable income in the preceding year, or you expect this year to be lower, you may use your current estimate as the basis instead. That's the SET method this whole guide describes.

So the practical rule for irregular income is:

• Expecting a similar or bigger year than last year? You can lean on last year's tax as a baseline, then revise up if needed.
• First full year freelancing, or expecting a quieter year? Use the estimate. Forecast this year's income and work from the SET.

Either way, the (A − C) ÷ B formula and the revision mechanism work the same. The estimate is just where your starting "A" comes from.

What if I under-estimate or pay late?

Two different things can go wrong, and they carry different consequences.

If you pay an instalment late (or pay less than the formula requires), Section 179(2) applies a 10% penalty on the unpaid portion if it isn't settled within 14 days of the due date. On top of that, interest runs at 1.5% per month on the outstanding amount from the original due date. This penalty is mechanical. It doesn't care why the payment was short.

If you simply under-estimated your income in good faith, the picture is gentler. Section 181(3) says no false-statement penalty applies where you "did not know and could not reasonably be expected to know" the estimate was wrong. An honest forecast that income later beat isn't treated as a false statement.

The lesson for irregular income is the same throughout: revise early and often. Every revised SET you file resets the remaining instalments to match reality, which is exactly what keeps you out of penalty territory.

How can I keep my estimate accurate without recalculating every quarter?

Doing this by hand means re-running the slab calculation, re-checking the 15% cap, and re-applying the (A − C) ÷ B formula every time a client pays. For income that swings around, that's a lot of spreadsheet work, and a small arithmetic slip can cost you a penalty.

The method matters more than the tool, though. Forecast the year, divide by four, and revise the moment your income outlook moves. Do that, and irregular income stops being a tax problem. It's just a cash-flow rhythm you've already planned for.

If you're still working out whether you need to pay quarterly at all, start with who must make quarterly payments. And if you're new to foreign-currency freelancing, the 2025 freelancer changes explain the income rules that feed into your estimate.