Guide · Savings & Investing
How to Work Backwards from a Savings Goal
Knowing you want to save £20,000 is not a plan — it is a wish. A plan specifies the monthly contribution, the account type, the interest rate, and the exact month you cross the line. Here is how to get there from the target.
Start with the end in mind
Every savings goal has four variables: your target amount, your timeline, your starting balance, and the interest rate you will earn. Fixing three of these immediately tells you the fourth. Most people fix the target and timeline, then calculate the required monthly contribution.
The four variables
Target amount · Timeline (months) · Starting balance · Interest rate (AER) → Monthly contribution required
Monthly contributions needed for common goals
The table below shows the monthly contribution required to reach each target, starting from zero and earning 4.5% AER (a reasonable 2026 easy-access rate):
| Goal | Timeline | Monthly needed |
|---|---|---|
| £5,000 (emergency fund) | 12 months | £407 |
| £5,000 (emergency fund) | 18 months | £268 |
| £20,000 (car / deposit top-up) | 3 years | £524 |
| £20,000 (car / deposit top-up) | 5 years | £305 |
| £50,000 (house deposit) | 5 years | £762 |
| £50,000 (house deposit) | 8 years | £461 |
| £100,000 (investment milestone) | 10 years | £810 |
Starting from £0. 4.5% AER compounded monthly. Figures are approximate.
The right account for the right goal
The account you choose should match the goal's timeline and your tax position:
- Under 2 years — Easy-access savings account. Flexibility matters more than maximising return. Interest stays within most people's Personal Savings Allowance (£1,000/year for basic rate taxpayers).
- 2–5 years — Cash ISA if you have ISA allowance available. Shelters interest tax-free and rates are competitive with standard easy-access accounts. A fixed-rate ISA can boost the rate if you will not need the money early.
- 5+ years — Stocks & Shares ISA. At this horizon, equity growth potential significantly outpaces cash interest rates in expected value, even accounting for short-term volatility.
Adjusting for a head start
If you already have savings set aside for the goal, your required monthly contribution falls. Existing savings compound on their own — they act as a foundation the monthly top-ups build on. For example, if you need £20,000 in 3 years and already have £5,000 saved:
| Starting balance | Monthly needed |
|---|---|
| £0 | £524 |
| £2,500 | £455 |
| £5,000 | £387 |
| £10,000 | £249 |
What if you cannot hit the monthly number?
Three levers: reduce the target, extend the timeline, or find a higher rate. Extending the timeline is usually the most powerful — going from 3 to 4 years on a £20,000 goal drops the monthly requirement from £524 to £389. A higher rate also helps but has diminishing returns: the difference between 4% and 6% AER on a 3-year goal is around £30/month.
Frequently asked questions
How do I work out how much to save each month?
Start with your target amount and deadline. Then account for what you already have saved and the interest rate you expect to earn. The formula is: monthly contribution = (target − (current savings × (1+r)^n)) ÷ (((1+r)^n − 1) ÷ r), where r is the monthly interest rate and n is the number of months. Our savings goal calculator does this automatically — just enter your target, timeline, starting amount, and expected rate.
Should I account for inflation in my savings goal?
Yes, for goals that are more than 2–3 years away. If you are saving for a house deposit of £40,000 and that goal is 5 years away, house prices may have risen by then. For goals priced in today's money, add the expected annual inflation rate to your return rate calculation, or increase your target amount to account for it. Short-term goals (within 12 months) do not need meaningful inflation adjustment.
What interest rate should I use for a savings goal?
Use the actual AER (Annual Equivalent Rate) of the account where you will keep the money. For cash in an easy-access account in 2026, that is typically 4–5%. For money invested in stocks and shares (appropriate for goals 5+ years away), a 5–7% real annual return is a reasonable planning assumption. Be conservative — it is better to save slightly more than to fall short.
Is a Cash ISA or easy-access account better for a savings goal?
For goals 3–5 years away, a Cash ISA is ideal if you have unused ISA allowance — it shelters the interest from income tax. For goals under 2 years, an easy-access account is fine since the interest earned is modest and falls within the Personal Savings Allowance (£1,000 for basic rate, £500 for higher rate taxpayers). For goals over 5 years, consider a Stocks & Shares ISA for better long-run growth potential.