요약
This calculator answers: “How long until I’ve saved enough for a deposit?” It accounts for your current savings, monthly contributions, compound interest growth, and the target deposit amount based on the property price and required deposit percentage.
The average UK first-time buyer deposit in 2024 was £61,090 (roughly 20% of the average purchase price of £292,000). While minimum deposits can be as low as 5% under the Mortgage Guarantee Scheme, most lenders prefer 10% or more for better rates.
작동 방식
The calculator simulates your savings growing month by month with compound interest. Each month:
- Your current balance earns interest:
balance × (1 + monthlyRate) - You add your monthly contribution:
balance + monthlyContribution - This becomes the new balance for the next month
This continues until your balance reaches the target deposit amount. The result tells you how many months (and years) it will take to hit your goal.
Why compound interest matters
Unlike simple interest (which only earns on the initial amount), compound interest earns on previous interest too. A 4% annual rate becomes 0.333% per month, applied to a growing balance. Over years, this compounds significantly — a £15,000 starting balance growing at 4% becomes £15,612 after one year, even without contributions.
공식
Where
The calculator repeats this formula until balance_n ≥ targetDeposit.
Minimum deposit requirements
- 5% deposit — available via Mortgage Guarantee Scheme (properties up to £600,000, permanent from July 2025)
- 10% deposit — most lenders’ standard minimum, better rates than 5%
- 15-20% deposit — unlocks significantly lower interest rates
- 25%+ deposit — access to best rates on the market
계산 예시
£300,000 property, 10% deposit needed
Target deposit (10% of £300,000)
= £30,000
Starting savings
= £15,000
Gap to fill
= £15,000 needed
Monthly rate (4% annual)
= 0.333% monthly
Month 1
= £15,550.00
Month 2
= £16,101.78
... continue until balance ≥ £30,000
= 28 months total
Result
Time to goal: 2 years 4 months (28 months)
입력값 설명
- Property price — the purchase price of the property you’re targeting
- Deposit percentage — the percentage of the price you need as a deposit (typically 5-20%)
- Current savings — how much you’ve already saved toward the deposit
- Monthly saving — how much you can add to savings each month
- Savings interest rate — the annual interest rate on your savings account (e.g., 3-5% for high-yield savings accounts in 2025/26)
출력값 설명
- Time to goal — how many months (and years + months) until you reach the target deposit
- Projected target date — the calendar date when you’re expected to reach the deposit amount
- Savings growth chart — a visual breakdown showing your balance growing over time, split between contributions and interest earned
Government schemes
Lifetime ISA (LISA)
- Save up to £4,000 per year
- Government adds a 25% bonus (up to £1,000/year)
- For first homes up to £450,000
- Must be aged 18-39 to open (can save until 50)
- 25% penalty if withdrawn for non-qualifying purposes (effectively loses bonus + interest)
Help to Buy ISA
- Closed to new applicants (November 2019)
- Existing holders can continue saving until November 2029
- Government adds 25% bonus (up to £3,000)
Mortgage Guarantee Scheme
- Permanent from July 2025
- Supports 95% LTV mortgages (5% deposit)
- Properties up to £600,000
- Does not provide funds — enables lenders to offer 95% mortgages with government backing
가정 및 제한사항
- The calculator assumes monthly compounding (most savings accounts compound monthly or daily; daily compounding gives slightly higher results but the difference is minimal)
- Interest rate is assumed constant — in reality, rates fluctuate with Bank of England base rate changes
- The calculator does not account for inflation — your purchasing power may decrease over time if interest rate < inflation
- Tax-free allowances are not modeled (but most savers won’t exceed the £1,000 personal savings allowance at typical interest rates)
- Lifestyle changes (salary increases, rent changes, unexpected expenses) are not predicted
- The calculator assumes you can consistently save the monthly amount without interruption
검증
All test cases hand-calculated using the month-by-month simulation formula above.
| Current savings | Monthly saving | Interest rate | Target deposit | Expected time | Source |
|---|---|---|---|---|---|
| £15,000 | £500 | 4% | £30,000 | 28 months | Hand calculation |
| £0 | £1,000 | 0% | £30,000 | 30 months | Hand calculation |
| £25,000 | £500 | 4% | £30,000 | 10 months | Hand calculation |
| £30,000 | £500 | 4% | £30,000 | 0 months (goal met) | Hand calculation |
| £5,000 | £800 | 5% | £50,000 | 52 months | Hand calculation |
Hand calculation for verification (£0 → £30,000, £1,000/month, 0% rate)
With no interest, the calculation is simple:
Months = £30,000 / £1,000 = 30 monthsThis matches the expected result above, confirming the simulation logic.
Sources
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