First-Time Buyer Calculator Methodology

What this calculator does

Takes property price, deposit, mortgage term, and interest rate. Returns Stamp Duty Land Tax (SDLT) due with first-time buyer relief, monthly mortgage payment, and total cost of purchase.

The formula

SDLT — First-Time Buyer Relief (England, from 1 April 2025):
  £0 – £300,000:          0%
  £300,001 – £500,000:    5%
  Above £500,000:         no FTB relief — standard SDLT rates apply

Standard SDLT rates (England, from 1 April 2025):
  £0 – £125,000:          0%
  £125,001 – £250,000:    2%
  £250,001 – £925,000:    5%
  £925,001 – £1,500,000:  10%
  Above £1,500,000:       12%

Mortgage monthly payment (standard repayment):
  M = P × [r(1+r)^n] / [(1+r)^n − 1]

Where:
  P = loan amount (purchase price − deposit)
  r = monthly interest rate (annual rate / 12)
  n = total months (years × 12)

Assumptions

SDLT rates are those applicable in England from 1 April 2025.
The buyer qualifies as a first-time buyer for SDLT purposes — the calculator does not verify eligibility.
The mortgage is a standard repayment (capital and interest) mortgage.
The interest rate is fixed for the full term of the calculation.
No other costs (solicitor, survey, moving) are included in the headline figure — these are noted as additional costs.

Data sources

Figure	Value used	Source	Last checked
FTB SDLT 0% threshold	£300,000	HMRC — SDLT first-time buyers relief	May 2026
FTB SDLT 5% upper limit	£500,000	HMRC — SDLT first-time buyers relief	May 2026
Standard 0% band	£0–£125,000	HMRC — Stamp Duty Land Tax	May 2026
Standard 2% band	£125,001–£250,000	HMRC — Stamp Duty Land Tax	May 2026
Standard 5% band	£250,001–£925,000	HMRC — Stamp Duty Land Tax	May 2026

Limitations

England only — Scotland uses Land and Buildings Transaction Tax (LBTT) and Wales uses Land Transaction Tax (LTT). Use the relevant calculator for those nations.
Assumes a single property purchase and confirmed first-time buyer status — the user must verify their own eligibility with HMRC or a solicitor.
Does not include solicitor fees, survey costs, or moving costs, which can add £2,000–£5,000 or more to the total purchase cost.
Does not model what happens at the end of any initial fixed-rate period — the monthly payment shown applies only at the interest rate entered.

Worked example

Inputs: first-time buyer, £280,000 property, £28,000 (10%) deposit, 25-year term, 4.5% interest rate.

SDLT: £280,000 is within the £300,000 FTB 0% threshold → £0 SDLT

Loan: £280,000 − £28,000 = £252,000
r = 4.5% / 12 = 0.375% = 0.00375
n = 25 × 12 = 300

M = £252,000 × [0.00375 × (1.00375)^300] / [(1.00375)^300 − 1]
  = £252,000 × [0.00375 × 3.0748] / [3.0748 − 1]
  = £252,000 × 0.01153 / 2.0748
  = £252,000 × 0.005557
  ≈ £1,400/month

Changelog

Date	Change
May 2026	Initial publication

Use the First-Time Buyer Calculator →

First-Time Buyer Calculator — Methodology