Why this page exists
A calculator is only worth using if you can see why its answer is right. This page describes how the tools here are built, checked and kept current, so you can judge the numbers rather than take them on faith.
Where the formulas come from
We do not invent methods. Each tool implements a formula that is published, standard and widely used, so that two people working carefully get the same answer:
- Loans use the standard annuity formula,
EMI = P × r × (1+r)n / ((1+r)n − 1), with a monthly rate and the tenure in months – see the EMI calculator. - Savings and investments use compound interest and annuity formulas, with the compounding frequency stated on the page: monthly for a SIP, quarterly for bank deposits, yearly for PPF.
- Income tax applies the published Indian slab rates, standard deduction, rebate and cess for the regime and year named on the page; GST uses the notified rate slabs.
- Energy needs use the Mifflin-St Jeor equation for basal metabolic rate with a standard activity factor – see the calorie calculator.
- Body measurements use the WHO body-mass-index bands on the BMI calculator and the US Navy circumference method for body fat.
- Pregnancy dates use Naegele's rule – 280 days from the first day of the last period, adjusted for cycle length – on the due date calculator.
The formula appears in plain text on the calculator's own page, with the meaning of each symbol and a worked example carried through to the final figure. That lets you reproduce the result on paper or in a spreadsheet, and catch us if we are wrong.
How a calculator is tested before it goes live
Nothing is published because the code looks correct. Each tool goes through three steps.
1. Hand calculation
We work through several examples by hand and compare them with what the tool prints – digit for digit, not "about the same". Rounding is checked too: rounding at the wrong step can shift an EMI by a rupee, or a tax figure by much more.
2. An independent re-implementation
The same calculation is written a second time, away from the website code, usually in a spreadsheet or a short script, and the two are run against a long list of inputs. Where they disagree, we find out which one is wrong before the page goes up.
3. Edge cases
Most wrong answers come from inputs nobody planned for. We feed each tool the awkward ones and require a clear message rather than a confident but meaningless number:
- zero, blank and partly filled forms;
- negative amounts, rates or ages, and very large values;
- a rate of exactly zero per cent, where the annuity formula divides by zero and the answer must be worked out differently;
- dates: 29 February, leap years, a loan starting on the 31st, and a future date in the age calculator;
- a tenure typed in months where the tool expects years.
If you see nonsense instead of a message, that is a defect – report it.
What changes over time
Some of these numbers are policy, not mathematics, and policy moves:
- Income tax slabs, deductions, rebate and cess – usually revised at the Union Budget.
- Small savings interest rates for PPF, Sukanya Samriddhi and similar schemes – reviewed every quarter. This is the routine update: most quarters nothing changes, and when it does, these are the pages that need touching, including the Sukanya Samriddhi calculator.
- GST rate slabs – changed whenever new rates are notified.
Wherever a tool uses such a figure, the page states the rate and the period it applies to, so you never have to guess which year's rules produced your answer. When a change is announced we re-check and update the affected calculators, and the "last updated" date at the foot of each page moves.
What these results cannot do
Every number here is an estimate for planning and comparison. The figure that counts belongs to someone else:
- Your bank or lender sets the actual EMI, with its fees and day-count convention.
- Your employer sets your real take-home pay.
- The tax department and your tax professional decide what you owe; our tax figure ignores anything we did not ask you about.
- Returns on market-linked investments are projections from the rate you typed, not a forecast. Nothing here is investment advice.
The health tools are general calculations, not a diagnosis and not medical advice. BMI, body fat and calorie figures say nothing about your particular health, and pregnancy dates are estimates a scan will correct. Talk to a qualified clinician before acting on any of it.
Telling us a number is wrong
We would rather hear it from you than leave it broken. Email support@codingmarble.com or use the contact page, with the calculator, the values you entered, and the answer you got against the one you expected.
We then re-do the hand calculation and the independent check for your case. If we were wrong, we fix the tool, refresh its "last updated" date and reply to say what changed. If it was right, we explain why.
The maths never leaves your device
Every calculation runs in your own browser. Your salary, loan amount, weight and period dates are not sent to us, not logged and not stored – no server does the sums, so there is nothing for us to keep. Our privacy policy covers what the site does collect, our terms of use the rest, and the about page says who we are.