How this calculator works
This calculator answers the two sides of the same question. "How long will it take?" projects your current saving pace forward until the goal is hit. "How much per month?" works backwards from a deadline to the deposit you need. Interest is compounded monthly, with deposits at the end of each month.
Try nudging the rate: at typical savings-account rates, the deposit does most of the work over short horizons, while over 10+ years compounding carries a surprisingly large share of the load.
The formula (deadline mode)
Required monthly deposit: PMT = (FV − P(1+i)^n) × i ÷ ((1+i)^n − 1)i = annual rate ÷ 12 · n = months · FV = goal · P = starting balance"How long" mode has no closed form once the balance grows month by month, so the calculator simulates until the goal is reached (capped at 100 years).
Worked example: $100,000 in 10 years
- Goal $100,000, starting balance $10,000, 6% annual rate compounded monthly, 120 months.
- The start grows on its own to 10,000 × 1.005¹²⁰ ≈ $18,194.
- The deposits must cover the remaining $81,806.
- PMT = 81,806 × 0.005 ÷ (1.005¹²⁰ − 1) ≈ $499 per month.
Frequently asked questions
What interest rate should I assume?
Use the rate you actually earn: a high-yield savings account rate for cash goals, or a conservative long-run estimate for invested money. Avoid planning around peak market returns — undershooting the rate beats undershooting the goal.
Does it account for inflation?
No — the goal is in today's dollars. For goals many years out, either raise the goal by expected inflation or use a real (inflation-adjusted) interest rate to keep everything in current purchasing power.
What if I already have more than the goal?
Time-to-goal is zero months. The calculator tells you that you are already there rather than projecting further growth.
Deposits at the start or end of the month — does it matter?
Start-of-month deposits each earn one extra month of interest, which shortens long timelines slightly. This calculator uses end-of-month deposits, matching how most people save right after payday arrives.
Can I use this calculator if I'm starting from $0 saved?
Yes — enter 0 as the starting balance and the calculator works the same way, using the same formula: PMT = (FV − P(1+i)^n) × i ÷ ((1+i)^n − 1). With P = 0, the required monthly deposit simplifies to the goal amount times i divided by ((1+i)^n − 1), meaning your deposits alone, plus their own compounding interest, have to cover the entire goal instead of just the remaining gap after a starting balance grows.