About Side by Side Calc

Side by Side Calc is a set of free calculators for eight money decisions people actually face: rent or buy, refinance or keep the current loan, a 15 or 30-year mortgage, lease or finance a car, Roth or traditional retirement contributions, an adjustable or fixed mortgage rate, an HSA or PPO health plan, and an electric or gas car. Each one runs both paths side by side from the same assumptions and ends in a single verdict, not just two numbers left for you to compare yourself.

Where the numbers come from

Every calculator uses a standard, published formula: the fixed-rate loan amortization formula for every mortgage and auto-loan comparison, the future-value-of-annuity formula for every "invest the difference" comparison, and standard deductible/coinsurance/out-of-pocket-max cost-sharing math for the health-plan comparison. The rent-vs-buy calculator uses a net-cost, opportunity-cost method in the spirit of the New York Times' own rent-vs-buy calculator. Nothing here is original research or a proprietary model -- each tool page names its method and shows the arithmetic behind the verdict.

What this site doesn't do

It doesn't store what you enter, require an account, or give tax, legal, or financial advice. Every calculator makes simplifying assumptions, stated on its own page -- the rent-vs-buy tool assumes the standard deduction unless you say you itemize, the ARM calculator asks for a single expected post-reset rate rather than modeling every adjustment cap, and so on. Real decisions have details these tools can't see; use them to understand the shape of a trade-off, then confirm specifics with a lender, tax professional, or plan document before acting.

Corrections

If you work in mortgage lending, tax, or benefits and a formula, rate, or assumption here is out of date or wrong, email hello@sidebysidecalc.com. Each page names its method so a correction can be checked directly against it.