The Section 24 calculator and finance-cost relief

The Section 24 Calculator is a what-if tool with a read-only live projection built from the transactions you have recorded. It helps you picture how residential finance costs affect your estimated tax. It does not file anything: your actual relief comes from recording mortgage interest as transactions, which flow through your quarterly updates and are finalised at year-end.

What the Section 24 calculator is

The Section 24 Calculator is two things on one page at /tax/section-24:

1. A read-only live projection at the top, computed automatically from the finance-cost transactions you have already recorded.
2. A what-if calculator below it, where you can type in figures by hand to model different scenarios.

Neither part submits anything to HMRC. The calculator is pure on-screen maths, and every number it shows is an estimate to help you understand the effect of residential finance costs (mortgage interest) on your tax. For the tax rule behind it (the basic-rate restriction on finance costs introduced from April 2020) and what it means for your situation, check with HMRC or your accountant, or see the MTD guide. The app states the rule in its What is Section 24? panel, but the rule itself is tax law.

Important: This page does not claim or file your finance-cost relief. Your actual relief is realised by recording your mortgage interest as transactions. See "How finance-cost relief is actually realised" below.

Who it is for and availability

This page is for property landlords. It is available to all signed-in property users on both Free and MTD Lite, and no HMRC connection is required to use it. (The nav entry is hidden for self-employment-only users.) Because nothing is submitted, you can use it before you have connected HMRC.

Read the live projection card

The live projection card sits at the top of the page and is built from your real transactions, so there is nothing to fill in.

1. Open the page. Choose Section 24 Calculator from the Tax & MTD nav, or go to /tax/section-24.
2. Read the projection. It shows your finance costs for the year so far, the relief applied (the app uses a 20% figure for the credit), and the amount carried forward to next year.
3. Follow the link if you want to act on it. The card includes a Go to quarterly submit link to /tax/submit, because that is where the recorded figures are actually sent.

Note: The projection soft-fails quietly if it cannot load (for example before you have recorded anything), and the what-if calculator below still works. A zero figure is not an error: relief can legitimately be zero until you have logged finance costs and recorded rental income.

Use the what-if calculator

The calculator lets you model a scenario by hand. Nothing you type here is saved or sent.

1. Enter your figures. Fill in Rental Income, Allowable Expenses, Mortgage Interest, and Other Income. Other Income is pre-filled from your saved tax settings, and you can change it for the scenario. To run, at least one of Rental Income or Mortgage Interest must be greater than 0.
2. Click Calculate Tax. The result shows your estimated tax due, the Section 24 Credit, the Finance costs carried forward, and a side-by-side comparison of Pre-2020 Rules against Section 24 (Current).
3. Click Reset to clear the inputs and model another scenario.

Tip: Treat every output as an estimate for planning, not a tax calculation you can file. The figures use illustrative rates and thresholds and do not replace HMRC's own year-end calculation.

How finance-cost relief is actually realised

This is the part that matters: the calculator never claims relief for you. Your relief is realised through your normal records.

1. Record your mortgage interest as transactions. Add each residential mortgage-interest payment against the property it belongs to, using the Residential Finance Cost category. Only date, amount, and category (plus the property) are required: a description and a receipt are always optional. See Recording income and expenses and Transaction categories.
2. Let it flow through your quarterly updates. Finance costs are carried automatically into your cumulative, year-to-date quarterly figures. You do not type a total anywhere. See Submit a quarterly update.
3. Finalise at year-end. Your figures are confirmed when you finalise the year and file your Final Declaration. See Year-End and your Final Declaration.

Important: Finance costs are not entered on the property annual finalisation page and do not auto-fill there. That page handles annual adjustments and allowances; finance costs travel through the quarterly cumulative updates instead.

If something goes wrong

• The live projection card does not appear or shows zero relief. Usually you have not logged any Residential Finance Cost transactions yet, or your rental profit caps the relief at zero. Record your finance-cost transactions and add your rental income, then reopen the page so the projection rebuilds.
• The calculator will not produce a result. It needs at least one of Rental Income or Mortgage Interest to be greater than 0. Enter a value and click Calculate Tax.
• The numbers do not match my expectation. The calculator uses illustrative rates and is an estimate only. Your authoritative position comes from HMRC's year-end calculation, and the figures it uses are built from your recorded transactions. Check that your finance-cost transactions saved against the right property with the right amount and date.

For anything about the Section 24 tax rule itself, your eligibility, or how much relief you are entitled to, check with HMRC or your accountant, or see the MTD guide.

Frequently asked questions

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