How this calculator works
A savings plan turns a regular deposit into a balance. The deposit can be daily, weekly, every two weeks or monthly, and the interest is applied at the same frequency. If you set a yearly increase, the deposit rises by that percentage at the start of each year, which models a plan that grows with your pay. In target mode the calculator works backwards: it runs the plan twice, once with no deposit and once with a deposit of 1, then scales the difference to find the first-year deposit that lands exactly on the target. Growth is linear in the deposit, so two runs are enough. The model assumes the rate is constant and that every deposit is made on time at the end of its period. A real plan may skip a month, change the rate, or pay a bonus, so treat the result as a guide rather than a promise.
Formula
Balance after n periods = B x (1 + r/m)^n + sum of each deposit grown to the end of the plan. A deposit in year y is D x (1 + g)^y, where D is the first-year deposit and g the yearly increase. Each deposit then earns the periodic rate for every remaining period. Interest = final balance - starting amount - total deposited. In target mode, first-year deposit = (target - balance with zero deposits) / (balance with a deposit of 1 - balance with zero deposits).
Worked example
Saving $1,000 to start, $100 a month for ten years at 4% a year gives a balance of $16,215.81, from $13,000 deposited and $3,215.81 of interest. The same plan with deposits rising 3% each year deposits $14,756.66 and ends at $18,207.98. To reach $25,000 on the level-deposit plan, the first-year deposit would need to be about $159.65 a month.
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Frequently asked questions
Does depositing more often help?
Yes, slightly. Money deposited earlier has longer to earn interest, so weekly deposits of the same annual total beat monthly ones. The difference is small at low rates but grows over long periods.
Why is the interest different from the rate times the balance?
Because the balance changes with every deposit and every interest credit. Interest compounds on money that is already there, so the total is not simply the closing balance times the rate.
Can I use a negative yearly increase?
The field does not allow it, because a plan with shrinking deposits is unusual. For a plan that stops, set the increase to zero and use a shorter horizon.
What about fees?
A fund fee reduces the return, so enter the rate net of fees. A 1% fee on a 5% return leaves about 4% for you, and over decades that difference is large.