Methodology

Why this page exists

A calculator that will not show its working is asking for blind trust. Every tool here displays its formula on the page, and this document explains the assumptions behind them.

Loans and mortgages

Monthly principal and interest use the standard amortisation formula:

M = P · r(1+r)ⁿ ⁄ ((1+r)ⁿ − 1)

where P is the amount financed, r the annual rate divided by twelve, and n the term in months. Where a rate is zero, the payment falls back to P ÷ n.

The mortgage tool adds property tax, home insurance and HOA dues to reach PITI — the figure a lender actually collects. When the down payment is below 20%, PMI is estimated at 0.55% of the loan per year; real quotes range from roughly 0.3% to 1.2% depending on credit profile and loan type.

Savings goals

Required monthly contributions come from the future-value-of-an-annuity relationship, accounting for what existing savings will grow to over the period. With no interest assumed, it reduces to the remaining amount divided by the number of months.

Energy costs

Running cost is watts × hours per day × days ÷ 1000 × your electricity rate. Enter the rate from your own bill; tariffs vary widely by region and time of use.

Materials — paint and concrete

Paint coverage is wall area × coats ÷ the coverage rating on the tin, then rounded up to whole containers, since paint is not sold by the fractional gallon. Concrete volume is length × width × depth converted to cubic yards, with a waste factor applied because over-ordering slightly is far cheaper than a second delivery.

Unit conversion

Length conversions normalise through a single base unit using exact factors — one inch is defined as 25.4 millimetres — so results do not drift through chained approximations.

How the maths is checked

Each formula is computed independently outside the page and compared against the value the page displays, including edge cases such as zero interest, zero quantity and boundary conditions. A result ships only when the two agree.

What we cannot model

Closing costs, escrow cushions, lender fees, regional tax rules, material price swings and site-specific complications are outside the scope of a general calculator. Treat every output as a planning figure to verify against real quotes.