Rates & sources
Standard amortisation formulas used across UK lenders. Interest rates move daily — confirm with your lender or broker.
Source: Bank of England — Statistics — figures refreshed at the start of each tax year.
When to use this calculator
- Before comparing lenders, brokers, or repayment options.
- When you want to test how a different deposit, rate, or term changes affordability.
- When you need a quick estimate before using a formal quote or agreement in principle.
- When you are stress-testing your budget against a potential rate rise to see the impact on monthly payments.
- When you want to understand the full cost of borrowing — not just the monthly figure — before you commit.
A realistic US planning example
Use these sample inputs as a quick scenario test, then change one variable at a time to compare outcomes.
Current Home Value ($)
$350,000
Outstanding Mortgage Balance ($)
$280,000
HELOC Interest Rate (%)
5%
Amount to Draw from HELOC ($)
50000
After entering these figures, review current home equity, equity % and current ltv together rather than in isolation — each metric tells a different part of the story. Then rerun the tool with one input adjusted to see which variable has the biggest effect on all three outputs before you settle on a plan.
How to read your results
Current Home Equity
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
Equity %
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
Current LTV
Loan-to-value helps you compare product eligibility and understand how much lender risk you are carrying at this deposit level. Crossing key LTV thresholds — typically 90%, 85%, or 75% — can unlock materially better interest rates.
Max HELOC Available
Use this metric to compare scenarios side by side and understand how changes in the key inputs drive the final outcome. If the figure surprises you, isolate one variable at a time and rerun the calculation to identify which assumption is responsible.
Monthly Payment
Use this to check whether the scenario fits comfortably within your regular budget. If it looks tight, rerun the tool with a longer term or larger deposit to find the boundary of affordability.
Total Interest
This shows the long-run cost of borrowing beyond the original principal, which is especially useful when comparing terms or weighing up overpayment options. A shorter term usually cuts this figure significantly even if the monthly payment rises.
Method & assumptionsAuthoritative sources
This calculator determines your current home equity and loan-to-value ratio, then estimates the maximum HELOC credit line your lender may offer based on the combined loan-to-value (CLTV) percentage you enter. The default 85% CLTV reflects the most common threshold used by major US lenders as of 2024, though some lenders allow up to 90% and others cap at 80%. Maximum HELOC availability equals (home value × max CLTV) minus your current mortgage balance. The monthly payment figure assumes the drawn amount is repaid in fully amortizing equal installments over the repayment period at the stated fixed rate; actual HELOCs typically carry variable rates tied to the Wall Street Journal prime rate, so real payments will fluctuate with rate changes.
The total interest calculation reflects the full repayment period and does not model the interest-only draw period that most HELOCs include before repayment begins, nor does it account for annual fees, closing costs, or inactivity fees that some lenders charge. Home value should reflect a realistic current market estimate, not the original purchase price or tax-assessed value. An independent appraisal or broker price opinion will be required by the lender and may differ from estimates produced by automated valuation models. Always obtain a formal Loan Estimate from your lender before making borrowing decisions.
Common mistakes
- !Mixing up loan amount and property value, which can distort affordability and LTV.
- !Using a headline rate but forgetting fees, insurance, taxes, or repayment type.
- !Testing only one term length instead of comparing the payment and total cost together.
- !Forgetting that a repayment mortgage and an interest-only mortgage produce very different monthly figures and total costs.
- !Not accounting for the impact of a rate revert after an introductory fixed period ends, which can sharply increase payments.
What to do next
- Run a second scenario with a higher rate or shorter term so you can see the downside clearly.
- Compare the result with an affordability or overpayment calculator before applying.
- Use the related guides below to understand trade-offs before you request live quotes.
- Note down the monthly payment and total interest for your two or three strongest scenarios so you have a clear comparison ready when you speak to a broker.
- Check whether making a modest overpayment each month would reduce total interest significantly — run the overpayment calculator next to find out.
Frequently asked
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End-of-article next steps
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