How we calculate

Updated 23 October 2026

Where do Instrukt's calculator figures come from?

From the published law and official data only: HMRC, the Welsh Revenue Authority, Revenue Scotland, the Finance Act 2003, the ONS, MHCLG and the Valuation Office. Each tool shows its sources and the date we last checked them, and we re-check after every Budget and before each tax year. The results are estimates, not advice.

Stamp duty, LTT and LBTT on a purchase

The stamp duty calculators apply each tax band by band: the rate for a band is charged only on the part of the price that falls in it, then the parts are added. Surcharges (additional property, non-UK resident, Scotland's Additional Dwelling Supplement) and first-time buyer relief are applied as each tax authority sets them out.

Sources: GOV.UK and HMRC (SDLT, England and Northern Ireland), the Welsh Revenue Authority (LTT) and Revenue Scotland (LBTT), linked on each tool.
Rates last checked: 2 November 2026.
Not covered: the less common reliefs and special cases, which each tool lists on its own page.

SDLT on a new commercial lease

The lease SDLT calculator works out the net present value of the rent as Finance Act 2003, Schedule 5 sets out: each year's rent is discounted at 3.5% a year, rent after year five is taken as the highest rent in the first five years, and the NPV is taxed at the non-residential lease rates. A premium is taxed separately and added. We test it against HMRC's own worked examples in its Stamp Duty Land Tax Manual.

Rates last checked: 2 November 2026.
Not covered: turnover and index-linked rents (enter an estimate), linked leases, overlap relief, agreements for lease, periodic tenancies and reliefs. England and Northern Ireland only (HMRC: leasehold purchases).

Rent review (CPI and RPI)

The rent review calculator multiplies the current rent by the change in the index between the two months you choose, then applies any collar, cap and upward-only rule you set. It uses the ONS's published series: CPI (D7BT, 2015 = 100) and RPI (CHAW, January 1987 = 100), monthly from January 2005.

Latest month held: September 2026, published by the ONS on 21 October 2026. We refresh the figures after the ONS's monthly inflation release.
Not covered: how your lease defines the review (which months, rebasing, compounding of a collar or cap). Check the clause itself; the tool follows the inputs you give it.

Business rates and the rateable value estimate

The business rates calculator works out a 2026/27 bill as the rateable value times the multiplier for its band, less small business rate relief in England. Wales uses its own multipliers.

If you don't have the rateable value, it can estimate one: floor area times the Valuation Office's average rateable value per square metre for that type of property in that council, on the 2023 list at 31 March 2026, uplifted by the published change at the 2026 revaluation. We don't look up individual properties: the rating list downloads are under a restricted licence, so we link to the official figure on GOV.UK instead.

Multipliers and reliefs last checked: 21 October 2026. Estimate data last checked: 21 October 2026.
Sources: MHCLG: 2026/27 business rates multipliers and budget package (information letter 5/2025); gov.uk: Estimate your business rates (2026/27 multipliers and worked examples); gov.uk: Small business rate relief; Business Wales: Non-domestic rates multipliers; gov.uk: Find a business rates valuation; Valuation Office: Non-domestic rating, business floorspace, March 2026 (RV per m² by property type and council); Valuation Office: Change in rateable value of rating lists, 2026 Revaluation, compiled list.
Refreshed each April for the new multipliers, and when the Valuation Office publishes new floorspace statistics.
Not covered: transitional relief, Supporting Small Business relief, charity, rural and empty property relief and other local reliefs. An estimated rateable value can be well off the real one for an unusual property.

Law firm start-up costs and statistics

The law firm start-up cost calculator uses the SRA's and the ICO's published fees for the practising year, and ranges for insurance, which depends on the insurer. The law firm statistics page reports figures exactly as the SRA, the Law Society, the Legal Ombudsman, the ONS and others publish them, each linked, and checked each quarter.

What every tool has in common

England and Wales law unless a tool says otherwise (the LBTT and stamp duty tools also cover Scotland).
The tools calculate; they don't advise. Instrukt is not a law firm or a tax adviser. For a decision that matters, ask a solicitor or accountant.
Found an error? Email hello@instrukt.co.uk. We correct it and change the tool's Updated date. See also our editorial standards.