"I want to save $20,000" is a wish. "I'll save $465 a month for three years" is a plan. Turning a goal into a monthly amount makes it something you can put in your budget and set up as an automatic transfer. Here's how the math works.
Start with the simple version
If your savings earn no interest, the math is just division:
Monthly savings = (goal − already saved) ÷ number of months
To reach $20,000 in 3 years (36 months) with $2,000 already saved:
($20,000 − $2,000) ÷ 36 = $500 a month
This is a safe upper bound. Once your savings earn interest, the monthly amount you need goes down.
Add interest
With interest, two things help you reach the goal:
- The money you've already saved keeps growing.
- Each monthly deposit earns interest from the day it's added.
The future-value formula puts both together:
Goal = saved × (1 + r)^n + monthly × ((1 + r)^n − 1) ÷ r
Here r is the monthly rate (annual rate ÷ 12) and n is the number of months. Solve for the monthly amount and you have your plan.
A worked example
Same goal as above, with the money in a savings account earning 4% a year:
- The $2,000 you already have grows to about $2,255 by the deadline.
- You need to save about $464.77 a month.
- Your total deposits come to about $16,732.
- Interest covers the remaining $1,268.
Interest cuts about $35 a month compared with the no-interest version.
Time helps more than you think
The longer your timeline, the less you need each month, and the more of the goal interest covers. Saving $20,000 from zero at 4%:
| Timeline | Monthly savings |
|---|---|
| 3 years | ≈ $524 |
| 5 years | ≈ $302 |
Starting two years earlier cuts the monthly amount by more than 40%.
Tips that make it stick
- Automate it: schedule the transfer for payday so the money is saved before you can spend it.
- Choose the right account: for goals under about five years, a high-yield savings account is steadier than the stock market.
- Plan for inflation: if the goal is years away, it will probably cost more by then.
Try it
The Savings Goal Calculator does this math for you. Enter your goal, what you've saved, your timeline, and your interest rate to see the monthly amount, your target date, and how much comes from interest. To see how a lump sum grows on its own, try the Interest Calculator. The Inflation Calculator shows how much a future goal will really cost.