Start with the saving decision
The tools in this section separate three different jobs: projecting a balance that receives deposits, finding the deposit needed for a dated target, and checking the terms of a cash account. Those jobs can use a percentage, but they do not use it in the same way. A contribution plan needs timing; a certificate needs a maturity date; an emergency reserve starts with expenses rather than return.
Use a compound-growth projection when the question is how a starting balance and recurring deposits might accumulate under one fixed rate. Use simple interest only when an agreement explicitly keeps interest from earning interest. The Rule of 72 is a fast scale check for a single growing balance, while the exact compound calculation is better when a date or a commitment depends on the answer.
Keep yield labels and access separate
APY is a one-year yield after the stated compounding convention; it lets deposits with different crediting schedules be compared on a common basis. It is not automatically the nominal rate, and it does not say that a rate will remain available. A CD calculator assumes money stays through maturity. A high-yield savings comparison assumes cash can move, but a real account may have rate tiers, promotional conditions, or transfer limits.
An emergency fund has a different purpose from either yield comparison. Its target comes from necessary monthly spending and a deliberate coverage period. A reserve intended for a job loss or urgent repair must be accessible when needed, so a higher return can be less useful if it locks cash or adds an early-withdrawal cost.
Use records that match the calculation
For any estimate, save the account balance, rate date, deposit timing, target date, and conditions that affect access. Run a lower-yield or missed-deposit case before relying on a long projection. These browser calculations make the arithmetic visible; the account agreement, statement, and personal cash-flow plan determine whether the assumptions are suitable.