A savings projection is only as useful as its assumptions
Understand return conventions, contribution timing, and what a model leaves out.
Returns are assumptions, not promises
A future-value calculator repeats the return pattern you enter. It does not know future market performance. A smooth 5% annual assumption creates a mathematical scenario, not a prediction that every year will actually deliver 5%.
Effective annual rates and monthly rates differ
On this site, an effective annual return is converted using (1 + annual rate)^(1/12) - 1. Dividing the annual rate by 12 would use a different convention. Check the convention when comparing results across websites.
Contribution timing changes the result
The monthly savings and investment tools add deposits at the end of each month after growth. A deposit at the beginning of the month would receive an extra month's growth. The money-lasting tool lets you choose start-of-month or end-of-month cash-flow timing.
Remember what is missing
Most scenarios here omit taxes, fees unless explicitly entered, changing contribution amounts, and market volatility. Retirement-plan projections do not enforce legal contribution limits or eligibility. Read the assumptions on the specific tool before relying on a number.
Compare more than one scenario
Changing one assumption at a time helps reveal which variables have the greatest influence. Try a lower return, a shorter time horizon, or a higher savings amount. These are sensitivity checks, not assigned probabilities.
Start with the Monthly Savings Calculator. For official background, see Investor.gov's compound interest tool.