Frequently Asked Questions
How the calculators work, where the numbers come from, what happens to the figures you type in, and how to use the tools on your own site.
Using the site
- Is SimplyCalculators free?
- Yes. Every calculator is free to use, with no account, no trial and no paywall. There is nothing to download and nothing to sign up for.
- Do I need to create an account?
- No. Nothing on the site is gated. You can open any calculator and use it immediately.
- Does it work on a phone?
- Yes. The site is built to work at phone width as well as on tablet and desktop, in any modern browser. You can add it to your home screen for one-tap access if you use it often.
- Does it work offline?
- Not reliably. The calculations themselves run in your browser, but the pages still need to load, so you need a connection to open one. Once a page is open, the calculator will keep working if the connection drops.
- How often is the site updated?
- New guides are published twice a week, on Wednesdays and Saturdays, and calculators are added and corrected on an ongoing basis.
Accuracy and methodology
- Where do the formulas come from?
- They are the standard published formulas for each calculation — Mifflin-St Jeor for metabolic rate, the amortisation formula for loan repayments, HMRC bands for income tax, and so on. Each calculator page sets out the method and the assumptions behind the result.
- How accurate are the results?
- The arithmetic is carried at full precision and rounded only for display, so the limiting factor is your inputs and the assumptions, not the computation. Results that depend on a published rate are only as current as that rate.
- Why does a figure differ from what I expected?
- Usually because a rate has moved or an assumption differs. Tax bands, energy prices, student loan thresholds and interest rates all change, and some calculators use defaults that may not match your circumstances. Check the stated assumptions first, then the current rate.
- Can I rely on these for professional or legal purposes?
- No. They are for general information and planning. Financial decisions, medical questions, and structural, electrical or gas work all need a qualified professional, and building work must meet Building Regulations regardless of what any calculator says.
- I think a calculator is wrong. How do I report it?
- Please get in touch through the contact page with the calculator name, the values you entered and the result you expected. Reports that include the exact inputs are the quickest to confirm and fix.
- Can you add a calculator that is missing?
- Suggestions are welcome through the contact page. Tools that solve a clear, common problem and can be explained properly are the ones most likely to be built.
Privacy and data
- Are the numbers I enter sent anywhere?
- No. Calculations run entirely in your browser, so the values you type are not transmitted to or stored on a server. Closing the tab discards them.
- Does the site use cookies?
- Yes. The site uses Google Analytics to measure how many people visit and which pages are used, and that sets cookies. It does not receive the values you enter into calculators. The privacy policy sets out what is collected and how.
- Do you sell data to anyone?
- No. See the privacy policy and GDPR pages for the full detail of what is collected, why, and the rights you have over it.
Reuse and embedding
- Can I put a calculator on my own website?
- Yes. Several calculators are available as embeddable widgets that you add with a single script tag. They are free to use, and the embed includes a visible link back to this site.
- Can I use the results in my own work?
- Yes, the results are yours to use. The guides and calculator pages themselves are copyright, so please link to them rather than reproducing the text.
- Are the calculators UK-specific?
- Some are. Income tax, VAT, salary and mortgage tools use UK rates and thresholds, and the rent and payslip guidance is written for UK tenancies and payroll. Converters, maths, health and science calculators are universal, though volume units default to US measures where US and Imperial definitions differ.
Financial calculators
- Do the tax calculators use current UK rates?
- They use the England, Wales and Northern Ireland bands: a £12,570 personal allowance, 20% basic rate to £50,270, 40% higher rate to £125,140 and 45% above that. Scotland sets its own bands and will give a different answer at the same salary.
- Why is my take-home pay lower than the calculator shows?
- Income tax and National Insurance are only part of it. Pension contributions, student loan repayments and payrolled benefits all come off too, and a non-cumulative emergency tax code will deduct more than the annual figures suggest.
- Is the mortgage calculator a quote?
- No. It applies the standard amortisation formula to the figures you enter, which tells you what a given rate and term cost. A lender quote also depends on affordability checks, fees, and the rate you are actually offered.
- Should I compare loans on monthly payment or total cost?
- Total cost. A longer term lowers the monthly payment and raises the amount repaid overall, so two loans with the same payment can differ by thousands. Compare the total interest, and check for early repayment charges before overpaying.
Math calculators
- Do the calculators show the working?
- Where the steps are the useful part — percentages, fractions, quadratics, statistics — the result is broken down rather than given as a single number, so you can check each stage against your own working.
- Which average should I use, the mean or the median?
- The mean when the data is roughly symmetric and totals matter; the median when it is skewed or contains outliers, such as incomes or house prices. The statistics calculator returns both so you can see how far apart they are.
- Does the trigonometry calculator use degrees or radians?
- You choose. Mixing the two is the most common source of a wrong trigonometric answer, so check the mode before assuming the formula is at fault.
- What is the difference between a permutation and a combination?
- Order. A permutation counts arrangements where the sequence matters, such as a PIN; a combination counts selections where it does not, such as lottery numbers. For the same figures the permutation count is always the larger.
Health and fitness calculators
- Is BMI a reliable measure of health?
- Only as a rough population-level screen. It uses height and weight alone, so it cannot distinguish muscle from fat and misclassifies very muscular people. Waist measurement and body fat percentage describe an individual better.
- How many calories do I actually need?
- Your BMR is what you burn at rest; multiplying it by an activity factor gives TDEE, the maintenance figure. The calculators use the Mifflin-St Jeor equation, which is an estimate with meaningful individual variation, not a prescription.
- Are these calculators medical advice?
- No. They apply published formulas for general information. Anything concerning a medical condition, medication, pregnancy or a significant change in diet or exercise should go to a qualified professional.
- How are heart rate zones worked out?
- From an estimated maximum heart rate, most commonly 220 minus your age. That formula carries a standard deviation of around 10 to 12 beats per minute, so a measured maximum from a field test is considerably more accurate.
Science and physics calculators
- Which units do the science calculators expect?
- SI units unless the input says otherwise — metres, kilograms, seconds, newtons, pascals and joules. Mixing units is the usual cause of an answer that is out by a factor of ten or a thousand.
- Do I need to convert temperatures to Kelvin?
- For gas law calculations, yes. Pressure and volume relationships are proportional to absolute temperature, so Celsius gives wrong answers and can produce a negative absolute temperature.
- How precise are the results?
- The arithmetic is carried at full floating-point precision and rounded for display. Your answer is only as good as your inputs, so apply significant figure rules to the measurements rather than to the number shown.
- Why does the molecular weight differ slightly from my textbook?
- Atomic masses are weighted averages of isotopes, and different sources round them differently. A discrepancy in the second decimal place is normal and rarely matters; a larger one usually means a miscounted subscript.
Utility and tool calculators
- Do the date calculators account for leap years?
- Yes, including the century rule: years divisible by 100 are not leap years unless also divisible by 400, which is why 1900 was not a leap year and 2000 was.
- Why does my file transfer take longer than the calculator says?
- Because broadband is sold in megabits and files are measured in megabytes, a factor of eight. Real transfers are slower again from protocol overhead, contention and the speed of the server at the other end.
- Do the converters use US or Imperial measures?
- US liquid measures where the two differ, which matters most for gallons and pints: an Imperial gallon is about 20% larger than a US one. Length, weight and area units are the same in both systems.
- What counts as a working day?
- Monday to Friday, excluding weekends. Public holidays vary by nation and year, so check them separately if a deadline depends on them.
Lifestyle calculators
- How do the sleep cycle times work?
- Sleep runs in cycles of roughly 90 minutes, and waking at the end of one feels easier than waking mid-cycle. The suggested times add whole cycles plus about 15 minutes to fall asleep, so treat them as targets rather than precise figures.
- Can I scale a recipe by simply multiplying everything?
- Ingredients scale linearly; cooking does not. Tin sizes, baking times and oven temperatures need adjusting separately, and raising agents and seasoning often need less than the multiplier suggests.
- How accurate is a pregnancy due date?
- It is an estimate from the last menstrual period assuming a 28-day cycle. Only a small minority of births happen on the predicted date, with most arriving within a fortnight either side.
- Is the dog age conversion really seven years to one?
- No. That rule of thumb is wrong at both ends. Dogs age very rapidly in their first two years and then more slowly, and larger breeds age faster than small ones.
Specialised trade calculators
- Should I add a wastage allowance to material quantities?
- Yes. Around 10% is usual for tiles and flooring laid straight, and more for diagonal patterns, awkward rooms or anything needing many cuts. Concrete is better over-ordered slightly than run short mid-pour.
- How do I measure an irregular room?
- Split it into rectangles, triangles and part-circles, calculate each separately, add them, then subtract voids such as a chimney breast or stairwell. Keep every measurement in the same unit before you start.
- Are these calculators a substitute for professional design?
- No. They size and estimate for planning and ordering. Structural, electrical and gas work must meet Building Regulations and should be specified and signed off by a qualified person.
- Why does my solar output not match the estimate?
- The figure assumes an orientation, tilt and irradiance for your location. Shading, panel temperature, inverter losses and dirt all reduce real output, and generation varies several-fold between summer and winter.
Browse the calculators
Each calculator also has its own page covering the method, the assumptions behind the result, and the questions specific to that tool.
Still have a question?
Send it through the contact page. If you are reporting a wrong result, including the calculator name and the exact values you entered makes it far quicker to confirm and fix.