Compare calculators

Most financial decisions are framed as binary: rent or buy, lease or own, this product or that one. The framing is usually wrong. The question is rarely 'which option is better' — it is 'better under what assumptions, by how much, and across what horizon'. A comparison calculator's job is to make those parameters visible.

These calculators take two or more candidate options and run them through the same cost-and-return engine, with the same time horizon, the same discount rate, and the same set of cost lines. The output is the gap between options, in dollars, across the entire horizon — not the gap on day one, which is the figure marketing materials lean on.

All maths uses 20-digit decimal precision. We make no jurisdiction-specific assumptions about taxes, transaction costs, or financing rates — you supply the local numbers. The output is a fair side-by-side comparison that respects the complexity of real decisions.

Making fair financial comparisons

A fair comparison has three properties: the same time horizon, the same set of cost categories, and the same discounting methodology. Comparing 'rent for 5 years' against 'buy and hold for 30 years' is not a comparison — it is two unrelated plans. The first rule of comparison: align the horizon.

The second rule is opportunity cost. The capital tied up in one option could have earned a return in another. A $100,000 deposit on a property is foregoing the long-run return that capital could have produced in a diversified portfolio — typically 5–8% per year nominal. Any honest comparison includes this opportunity cost as an explicit line in the cost stack.

The third rule is time value of money. A $5,000 cost in year 30 is not the same as a $5,000 cost today. Discounting cash flows to present value at a consistent rate is the standard tool for collapsing a multi-year stream of costs into a single comparable number. Most consumer comparison content skips this step entirely — and produces conclusions that change once the discount rate is added.

Rent vs buy decisions

The rent-vs-buy decision is the most common high-stakes financial comparison most people face. Done well, it considers: deposit (capital tied up), mortgage interest, property taxes, insurance, maintenance, body-corporate fees, transaction costs (stamp duty, agent fees), expected capital growth, and the rent path of the equivalent property.

The most common mistake is comparing 'mortgage payment' to 'rent payment'. This compares the wrong numbers. The right comparison is 'total cash outflow under buying' (mortgage + holding costs + transaction amortisation, less expected sale proceeds) against 'total cash outflow under renting' (rent + opportunity cost of the deposit invested elsewhere). The gap between the two — across a realistic 7-15 year horizon — is the answer.

The rent-vs-buy calculator runs the comparison with full cost stacks and an opportunity-cost line. The break-even year — where buying overtakes renting in cumulative cash terms — typically lands between year 5 and year 10 depending on rent path, growth assumption, and rate. If you plan to move within the break-even year, buying loses; if you plan to stay beyond it, buying wins.

Product and asset comparison

Comparing two consumer products on lifetime cost is more useful than comparing them on purchase price. A $1,500 appliance with a 15-year life and $50/year running cost has a lifetime cost of $2,250; a $700 appliance with a 5-year life and $80/year running cost has a lifetime cost of $1,100 across 5 years — but $3,300 across 15 years if you buy three of them. The comparison flips depending on horizon.

The lifetime-cost calculator extends this logic to any consumer good: appliance, vehicle, professional tool, software stack. The output is total cost over a configurable horizon — including expected replacement cycles, maintenance, and any utility loss as the product ages.

Asset comparison goes further. The asset-comparison calculator runs two or more investments through the same return-and-cost projection. The lifecycle-cost variant adds the operational cost of owning the asset across its useful life. Both are useful for any decision involving capital allocation between competing options that promise different returns at different risk and cost profiles.

Comparing savings and yield products

Savings products quote nominal interest rates; the comparison number for any decision is real net return — nominal rate minus inflation minus fees minus tax on interest. A 4% savings account in a 3% inflation environment with 0.2% fees and 25% marginal tax on interest delivers a real net return closer to 0.6% than the headline 4%. The rate that matters is the one that survives all four drags. Most marketing materials quote the nominal rate; the savings-account-comparison calculator runs the real net calculation directly.

The promotional-rate dynamic is the second trap. A savings product offering '5% for the first 6 months, 1% thereafter' has a blended yield much closer to 1% than 5% over any horizon longer than the promo period. A 12-month deposit of $20,000 at the 5%-promo-then-1% structure earns roughly $600 in interest across the year, not the $1,000 the headline suggests. The savings-account-comparison calculator combines bonus and steady-state rates across the holding horizon so the comparison number reflects what the saver will actually earn, not what the marketing emphasises.

Any savings product competes against alternatives: short-term bonds, money-market funds, even the marginal repayment on outstanding debt. A savings account paying 3% real net return is a poor choice when an outstanding consumer loan charges 8% — the marginal dollar pays down the loan at a guaranteed 8% return, free of credit risk and tax. The right comparison is 'this product vs the next-best alternative use of the same dollar', not 'this product vs zero return'. The real-return calculator and the interest-saved calculator together cover both ends of the alternative-use spectrum.

Scenario modelling

The future is not a single forecast — it is a distribution of scenarios. A retirement projection that assumes 6% returns is one scenario among many; the same projection at 4% or 8% produces materially different terminal balances. Sensitivity analysis is the discipline of testing which assumptions matter most.

The investment-scenario-comparison calculator runs the same portfolio through multiple return assumptions side-by-side. The gap between scenarios reveals which inputs are load-bearing: for long-horizon projections, the return rate dominates contributions; for short horizons, contributions dominate return rate. Knowing which axis to push on is more useful than the point-estimate forecast itself.

The scenario-builder calculator extends this to any financial decision: career change, business launch, large purchase, portfolio reallocation. Build out three scenarios — base, optimistic, pessimistic — and run them through the same horizon. The point is not to find a single answer; it is to know what you are exposed to under each scenario, and to make a decision that survives the pessimistic case.