Savings Calculator

How to Use the Savings Calculator

This is the tool for the question "if I put aside this much every month, where do I end up?" It compounds monthly, which matches how nearly every savings account works.

1. Enter what you already have. Leave it at zero if you are starting fresh — the calculation still works, and the interest slice will simply be smaller for the first few years.
2. Set the monthly deposit. Use the amount that actually leaves your account by standing order, not the amount you hope to save. The budget calculator is the honest way to find that figure.
3. Enter the rate. Savings accounts usually advertise an APY, and entering it here is close enough for planning — the difference against the nominal rate is a few pounds over a decade.
4. Set the horizon. Drag the years slider and watch the interest share tile. Under five years it is usually below 10%; past twenty it starts to dominate.

The yearly table separates what you deposited from what the account added, so you can see the exact year at which the account starts contributing more than you do.

Two habits make the projection more useful. First, run it twice — once at the rate you are getting and once two percentage points lower — so you have a floor as well as a target. Second, revisit it whenever your deposit changes rather than at year end, because a deposit increase applied early is worth considerably more than the same increase applied late.

Savings Growth Formula

A savings balance is a lump sum and an annuity added together, both compounded at the monthly rate.

Deposits are treated as arriving at the end of each month. If your standing order goes out on the first, you will do fractionally better than this projection — about 0.3% more over ten years at 4%.

Example

$2,000 to start, $250 a month, 4% for ten years

1. Monthly rate: 0.04 ÷ 12 = 0.00333333. Months: 12 × 10 = 120.
2. Growth factor: 1.00333333¹²⁰ = 1.490833.
3. Starting balance grows to 2,000 × 1.490833 = $2,981.67.
4. Deposits grow to 250 × ((1.490833 − 1) ÷ 0.00333333) = 250 × 147.2499 = $36,812.45.
5. Final balance: 2,981.67 + 36,812.45 = $39,794.12. You deposited 2,000 + 30,000 = $32,000, so $7,794.12 is interest — 19.6% of the balance.

What one extra year does

Run the same numbers for eleven years instead of ten and the balance reaches $44,471. The extra year adds $3,000 of deposits and $1,677 of interest. Run it for twenty years and the balance is $96,139, of which $34,139 is interest — the interest share has climbed from 19.6% to 35.5%. Nothing changed except time.

Reading the Split Between Deposits and Interest

The useful way to read a savings projection is by asking which slice is doing the work.

For the first five years, saving is almost entirely a matter of discipline — the interest is a rounding error against the deposits. Somewhere around year seventeen the account is adding more each year than you deposit, and from there the balance climbs faster than your discipline does. That crossover is why "start early" is advice rather than a platitude.

It also tells you where to spend your attention. In the first few years, increasing the deposit moves the needle far more than chasing a better rate. Later, the rate matters more, because it is being applied to a much larger balance.

Here is the arithmetic behind that. In year one of the example, a $50 increase in the monthly deposit adds $611 to the balance while a full extra percentage point on the rate adds about $35. By year twenty the same $50 increase is worth roughly $18,339 and the same percentage point about $12,045 — the rate has closed most of the gap but not all of it. The practical conclusion is that deposit size is the lever you control directly and should pull first, while rate shopping is worth doing once a year rather than agonising over.

What the Projection Leaves Out

A few things this calculator deliberately does not know about.

Tax. Interest is taxable income in most places unless the account is sheltered. If you pay 20% on savings interest, enter 3.2% rather than 4% and the projection becomes an after-tax one.

Rate changes. Savings rates are not fixed. Introductory bonuses expire, and variable rates follow central bank decisions. Assume today's rate holds and you will be wrong in both directions over a decade — which is another argument for running a pessimistic scenario alongside the optimistic one.

Inflation. A 4% return with 3% inflation is a 1% real return. Your balance grows and your buying power barely moves. For anything longer than about five years, check the result against the inflation calculator before deciding the number is good news.

Two practical suggestions. Keep an emergency fund in instant access before locking anything into a fixed-term account, because breaking a fixed term usually forfeits the interest that made it worth choosing. And once your emergency fund is complete, consider whether a savings account is still the right home — over twenty-year horizons, the difference between a savings rate and a long-term investment return is the difference between $96,139 at 4% and $138,309 at 7% on identical deposits.

Finally, watch the shape of the account itself. Many headline savings rates apply only up to a balance cap, or only for the first twelve months, or only if you pay in a minimum each month and make no withdrawals. A projection built on the headline rate that then quietly earns the reversion rate for nine of its ten years will overshoot badly. The safest approach is to model the reversion rate and treat any bonus period as an unbudgeted extra.