Estimating Your Tax Bill Under MTD (So There Are No Surprises)
How to estimate your income tax and National Insurance through the year using your MTD quarterly figures, why quarterly updates don't calculate tax, and a simple set-aside rule to avoid a January shock.
The short answer
MTD quarterly updates report income and expenses but don't calculate your tax — that happens at the final declaration. To estimate your bill through the year, take your cumulative profit, subtract your personal allowance, apply the income tax bands and Class 4 National Insurance, and set aside a percentage of every payment (often 20–30%) so the January bill is already covered.
One of the best things about MTD is that you have up-to-date figures all year. Used well, that means no January nasty surprise. Here's how to estimate your bill as you go.
Quarterly updates don't calculate tax
First, a myth to clear up: your quarterly updates don't work out your tax. They report cumulative income and expenses. The actual tax and National Insurance are calculated at the final declaration after year-end. But you can estimate as you go using the same figures.
The estimate in five steps
Using your cumulative profit so far:
- Start with profit (income minus allowable expenses).
- Subtract your personal allowance (the tax-free slice).
- Apply the income tax bands to what's left.
- Add Class 4 National Insurance on profits above the threshold.
- Remember any payments on account already made.
Because MTD figures are cumulative, you can redo this each quarter to see the bill building.
A simple set-aside rule
You don't need perfect maths — you need a buffer. A common approach:
| Profit level | Rough set-aside |
|---|---|
| Lower earners | ~20% |
| Middle earners | ~25% |
| Higher earners | 30%+ |
Move that percentage of every payment into a separate tax pot the day it lands. When the bill comes, it's already covered — and anything left over is a bonus.
Don't forget payments on account
If your bill is over £1,000 and not mostly taxed at source, you'll make payments on account — advance instalments that can make your first January feel like 150% of a normal bill. It's timing, not extra tax, but plan for it — see our payments on account guide.
Let software do the heavy lifting
Many MTD tools — and HMRC's own calculation — give an estimated liability from your figures. Treat it as a guide to how much to set aside, and confirm the exact number at the final declaration.
ThisQuarter can trigger and retrieve HMRC's own tax calculation as part of the year-end flow, so the figure you finalise on is HMRC's, not a guess.
Key takeaways
- Quarterly updates report figures; tax is calculated at the final declaration.
- Estimate from cumulative profit: minus allowance, apply bands, add Class 4 NIC.
- Set aside 20–30% of every payment into a separate tax pot.
- Plan for payments on account so your first January doesn't bite.
This article is general information, not tax advice. Check HMRC's guidance and tax rates at gov.uk or speak to your accountant.
Frequently asked questions
Do MTD quarterly updates tell me my tax bill?
Not exactly. Quarterly updates report your cumulative income and expenses, but the tax and National Insurance are calculated at the final declaration after year-end. Some software gives a running estimate, but it's an indication, not the final figure.
How much should I set aside for tax?
A common rule of thumb is 20–30% of your profit, depending on your income level and whether payments on account apply. Higher earners should lean toward 30%+. Setting aside from every payment is safer than finding a lump sum in January.
Why is my first January bill so big?
Because of payments on account — you can pay last year's tax plus the first instalment of next year's in one go. It's timing, not extra tax, but it's why estimating and saving through the year matters.
Can software estimate my tax for me?
Yes — many MTD tools, and HMRC's own calculation, give an estimated liability from your figures. Use it as a guide to how much to set aside, and confirm the real number at the final declaration.
Related reading
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