The short answer: work backward from a date
"How much should I save each month?" has two kinds of answers. The first is a rule of thumb — save 10 to 20 percent of your income, or follow the 50/30/20 split. The second is exact math for a specific goal, and it is more useful: a goal with a date turns into a monthly number you can actually put on autopilot.
The simple version needs nothing but division. Take the goal, subtract what you have already saved, and divide by the months until the date:
Want $6,000 for a trip in 18 months with nothing saved yet? That is $6,000 ÷ 18 = $333.33 a month. The moment a vague wish becomes "$333 a month," it becomes a plan — you can compare it against your budget and decide if the date, the goal, or your spending has to move.
The precise version: let interest do some of the work
Simple division ignores two things working in your favor: money you have already saved keeps growing, and every deposit earns interest for the months that remain. The interest-aware formula accounts for both, so its answer is always a little lower — free money from compounding.
Here r is the monthly interest rate (your APY divided by 12, then by 100) and n is the number of months. In words: grow your current savings forward, subtract that from the goal, and spread what remains over deposits that each earn interest too.
Worked example: $6,000 in 18 months at 4% APY
You have $500 saved toward a $6,000 vacation fund, 18 months to go, and a 4% APY account.
Growth factor: (1.003333)18 ≈ 1.0618
Saved amount grown: $500 × 1.0618 = $530.88
Still needed: $6,000 − $530.88 = $5,469.12
Monthly deposit: $5,469.12 × 0.003333 ÷ 0.0618 = $295.32
Total deposited: $500 + ($295.32 × 18) = $5,815.81
Interest earned: $6,000 − $5,815.81 = $184.19
Answer: $295.32 a month — about $38 less than the $333.33 that plain division suggested. Your head start and 18 months of compounding cover the difference.
The emergency-fund version of this math
Emergency funds use the same formula with a goal derived from your spending, not a price tag. The standard guidance is three to six months of essential expenses — housing, utilities, groceries, insurance, transport, minimum debt payments — not your total spending. If your essentials run $3,200 a month, the target band is $9,600 to $19,200.
That band looks intimidating, so build it in layers: a $1,000 starter cushion first (enough to keep a surprise bill from becoming new debt), then one month of essentials, then keep going. Each layer gets its own monthly number from the formula above — and each one is a finish line you can actually see.
When the monthly number is too big
Sometimes the formula hands you a number your budget laughs at. That is not failure — it is information. You have exactly three levers, and the honest move is to pick one deliberately:
- Extend the timeline.
More months is the gentlest lever: it both spreads the goal thinner and gives compounding more time. Pushing the $6,000 example from 18 to 24 months drops the deposit to about $225. - Trim the goal.
A $4,500 trip you actually take beats a $6,000 trip you never fund. Shrink the target until the monthly number fits. - Raise the monthly amount.
Cancel a subscription, sell something unused, or bank the next raise instead of spending it. Even $50 more a month shortens the timeline visibly.
Run the numbers for each lever before choosing — guessing which one helps most is exactly what the calculator is for.
What the formula assumes
The math describes steady monthly deposits into an account earning a fixed APY — no withdrawals, no rate changes, no fees. Real accounts vary, but for planning purposes the approximation is close: savings rates move slowly, and the formula's real job is turning a goal into an actionable monthly habit, not predicting the balance to the cent. Inflation and taxes on interest are also outside the formula, so think of the answer as the account balance, not purchasing power.
Try it: plug your own numbers into the free savings goal calculator — flip between "monthly needed" and "when will I reach it" to check your plan from both directions, and watch the growth chart draw your path to the goal.