Online SDLT calculators have proliferated over the last decade. Search "stamp duty calculator" and you'll find offerings from estate agents, mortgage brokers, banks, conveyancing firms, and HMRC itself. They all do essentially the same thing — take a purchase price, ask a couple of yes/no questions, and apply the standard band table. For the majority of straightforward purchases, this is enough. For a meaningful minority, the answer is wrong.
What a typical calculator does well
A good calculator implements the post-April-2025 bands correctly: 0% on the first £125,000, 2% from £125,001 to £250,000, 5% from £250,001 to £925,000, 10% to £1.5m, and 12% above. Most also handle the first-time buyer relief correctly (0% to £300,000, 5% to £500,000, no relief above) and the 5% additional-dwelling surcharge for second homes. For a standard purchase by a UK-resident individual with no complications, any decent calculator will give the right answer.
Where calculators commonly go wrong
Chattels. Almost no general-purpose calculator subtracts the value of chattels (carpets, curtains, white goods, freestanding furniture) from the purchase price before calculating SDLT. The standard approach is to take the full purchase price at face value. If your purchase contract or TA10 form includes meaningful chattel values, the SDLT on the "net of chattels" price will be lower. Our refund estimator does account for this; HMRC's online calculator does not.
Multiple dwellings on a single transaction. Before 1 June 2024, Multiple Dwellings Relief allowed a meaningful SDLT reduction on purchases that included a self-contained annexe (granny flat) or two or more dwellings. Calculators that don't flag this option will overstate SDLT for affected pre-June-2024 purchases. MDR is now abolished going forward, but historical purchases may still be entitled to refunds.
Mixed-use property. If a property includes a commercial element — a working shop on the ground floor, a paddock used for grazing, a working farm — the non-residential SDLT rates apply (much lower). Calculators that ask only about "residential vs commercial" and don't handle mixed cases tend to default to residential, overstating the bill.
Non-resident surcharge. Non-UK-resident buyers pay an additional 2% surcharge on top of all other SDLT charges. Many calculators don't ask about residence status. See our non-resident surcharge guide.
Surcharge stacking. A non-resident buyer purchasing a second home pays standard SDLT plus the 5% additional-dwelling surcharge plus the 2% non-resident surcharge — all applied to the full purchase price. The combined effect on a £750,000 purchase is around £80,000. Calculators that handle one surcharge but not the other underestimate the bill.
HMRC's own calculator
HMRC's online tool produces a correct standard calculation but does not adjust for chattels and gives only limited support for unusual cases. It tends to overstate SDLT for buyers with chattel-rich purchases. See our piece on why HMRC's calculator overcharges.
The buyer's checklist before relying on a calculator
Before trusting a calculator's output, ask yourself: Am I buying as a UK resident? Do I own (or am I about to own) any other residential property worldwide? Am I a first-time buyer who has never owned anywhere before? Does the property contain a self-contained annexe? Does it have any commercial element? Are there meaningful chattels included? Am I buying through a company?
If you answered "no" to all the complicating questions, a standard calculator is usually fine. If you answered "yes" to any, the standard calculator is probably wrong — and the direction of the error is almost always: it overstates what you actually owe.
A worked example
A £500,000 purchase by a UK-resident upgrader (already a homeowner). Generic calculator answer: standard SDLT of £15,000 + 5% surcharge of £25,000 = £40,000 SDLT. Now suppose £8,000 of chattels are itemised on the TA10 (carpets in three rooms, white goods, garden equipment, freestanding wardrobes). Adjusted price: £492,000. Standard SDLT on £492,000 = £14,600. Surcharge on £492,000 = £24,600. Adjusted SDLT = £39,200. Saving: £800. Modest at this price point, but pure overpayment if not claimed.
The same exercise on a £1.2m purchase with £15,000 of chattels can save closer to £1,800. On a £2m+ purchase with substantial chattels and a self-contained annexe before June 2024, the saving can run to five figures.
Use the right calculator for your case
Our SDLT calculator handles standard purchases plus surcharges. Our refund estimator layers in chattels and helps you assess overpayment. Use both for a complete picture.
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