In-depth guides that put two financial choices next to each other — same numbers, same assumptions, different decision. Each comparison links to the matching live calculator so you can run the question on your own figures.
Personal-finance decisions almost always reduce to a comparison: this option vs that option, this term vs that term, this account type vs that one. Should you take the 30-year mortgage with the lower payment, or the 15-year with the lower total interest? Should you put the extra £200 a month into the ISA or the SIPP? Is the personal-loan APR cheaper than rolling the balance onto the 0% credit card? These questions sound straightforward but they all turn on numbers that interact — tax wrappers, compounding frequency, opportunity cost, fees, and time horizon — and the wrong answer usually comes from a calculator that only compares one of the variables.
A good side-by-side comparison runs both options against the same set of inputs and shows the result. Same balance. Same interest rate. Same time horizon. Same tax treatment where it applies. Only one variable changes at a time. That's the format the articles here use. Each one takes one choice with two (or sometimes three) sensible answers, lays out the math for each path, runs a worked example with realistic figures, and shows the lifetime difference. Where the right answer depends on a variable the reader controls — your marginal tax rate, your expected return, your time to retirement — the article makes that explicit instead of hiding it behind an "average" that suits no-one.
Each comparison also links to the matching live calculator, so once you've understood the math you can swap our example numbers out for your own. The articles are the explanation; the calculators are the tool. Both are free, both run in your browser, and neither tries to sell you a product at the end.
Compound earns interest on interest; simple earns only on principal. See the long-term gap with worked examples + a live calculator.
Lower monthly payment vs lower total interest. Run both scenarios on the same loan amount and see the lifetime cost difference.
The gap between compound and simple interest is small over a single year and enormous over thirty. Simple interest pays a fixed return on the original principal every year; compound interest pays a return on the principal and on every dollar of interest already earned. A £10,000 deposit at 6% simple interest grows to £16,000 after ten years. The same deposit at 6% compounded monthly grows to £18,194. Push the horizon to thirty years and the simple-interest balance is £28,000 while the compound balance is £60,226 — more than double. The guide walks through the formula (FV = PV(1 + r/n)n·t for compounding, FV = PV(1 + r·t) for simple), where each one is actually used in the real world (compounding for savings accounts, mortgages, and investments; simple interest for some short-term loans and some bonds), and why the compounding frequency matters almost as much as the rate.
The mortgage-term trade-off is the cleanest example of a decision that depends on what you do with the difference. A 15-year mortgage on a $400,000 loan at 6% costs roughly $3,375 a month and $207,000 in total interest. The 30-year on the same loan at 6% costs roughly $2,398 a month and $463,000 in total interest. The 15-year wins on lifetime cost by $256,000 — but only if the cash saved on the smaller payment on the 30-year option isn't invested. Invest the $977-a-month difference at a 7% expected return for thirty years and the 30-year mortgage plus invested difference comes out ahead. The guide runs the worked example with the same loan amount on both terms, shows the lifetime cost difference, runs the "invest the difference" comparison both with and without realistic tax treatment, and explains the cases where the 15-year still wins regardless (most importantly, behaviourally: forced saving via the higher mortgage payment beats discretionary investing for most households).
The comparison guides we're working on next, in roughly the order they're likely to land. If one of these is the question you're asking right now and you'd like the article sooner rather than later, email [email protected] — the order changes based on which guides readers actually want.
A side-by-side comparison is only useful if you read it knowing which numbers in the article matter for your situation and which are just illustrative. The guides on FinCalcHub follow a consistent pattern, so you can pick the relevant bit out fast:
One last thing worth saying about how to use these guides: they are not advice. They are the math, the worked example, and an explanation of what swings the answer. The decision belongs to you, and if your situation involves a wrinkle the article doesn't cover — cross-border tax, an unusual pension arrangement, an estate-planning angle — that's the point where a chartered tax adviser or chartered financial planner earns their fee.
If you're not sure which comparison applies to your question, start with the relevant topic: