Pick the number, then the tool
Tool names overlap. Percentage, Percentage Increase, and Discount all divide one number by another. Loan, Mortgage, Investment, and Retirement all raise (1 + r) to a power. Start from what you have in hand and what you want back, then follow the row.
Money
15 toolsPercentages, tax, pay, and anything with a rate raised to a power.
- PercentageX% of Y, X as a share of Y, or change from A to B → three modes on one page
- Percentage Increaseold value, new value → rise or fall as a percent, sign kept
- Discountlist price, percent off → sale price, amount saved
- Sales Taxprice, tax rate → tax owed, total, or the pre-tax figure from a receipt
- Tipbill, tip percent, head count → tip and per-person split
- Salary to Hourlyannual pay, hours per week → hourly, daily, weekly, monthly
- Simple Interestprincipal, rate, years → interest with no compounding
- Compound Interestprincipal, rate, years, compounding frequency, monthly deposit → balance by year
- Investmentstarting balance, monthly contribution, return → pre-tax balance, year by year
- Loanamount, rate, term in months, optional extra payment → monthly payment, total interest
- Mortgagehome price, down payment, rate, term → monthly principal and interest
- Retirementage, savings, contribution, return → balance at retirement age
- Millionairesavings, monthly deposit, return → years until 1,000,000
- Cost Comparisontwo options, price and quantity each → cost per unit
- Black Fridaypre-sale price, deal price, bundle size → the real saving per unit
Body
10 toolsScreens and estimates. Every one of these has a published error band.
- BMIweight, height, metric or imperial → BMI and the WHO band
- Ideal Weightheight, sex, frame → target range from four formulas
- Body Fattape measurements → US Navy estimate
- Harris-Benedict BMRweight, height, age, sex → resting calories per day
- CalorieBMR inputs plus activity level → daily intake to lose, hold, or gain
- Steps to Caloriessteps, weight, pace, terrain → calories burned
- Sleepwake time → bedtimes counted back in 90-minute cycles
- Pregnancyfirst day of last period → due date, current week, milestones
- Korean Agebirth date → Korean age beside international age
- Coffee Kickdose, body weight, time drunk → caffeine peak and clearance
School
3 toolsWeighted averages. The scale is the input people forget to check.
- Gradecategory weights, scores so far → current course grade and the score each letter needs on the final
- Test Gradequestion count, wrong answers → percent and letter for every score
- GPAletter grades, credit hours → weighted or unweighted GPA, cumulative across terms
Physics and chemistry
7 toolsTextbook equations. The units on each box matter more than the equation.
- Kinetic Energymass, velocity, in any of the listed units → ½mv² in joules, kWh, BTU, or six others
- Free Fallheight or time → impact speed and fall time under g
- Exponential Decayhalf-life or decay constant, elapsed time → fraction remaining
- Titrationmolarity and volume of one solution → the unknown at the equivalence point, working shown
- Schwarzschild Radiusmass → event horizon radius
- Black Hole Collisiontwo masses → merged mass and energy radiated as gravitational waves
- Car Crashvehicle mass, speed before and after, impact time → impact force and energy
Kitchen, weather, and holidays
9 toolsRules of thumb with a formula behind them. Expect a range, not a decimal.
- Pizza Sizediameter and price, two pizzas → area and cost per square inch
- Ideal Egg Boilingegg size, start temperature, altitude → minutes for the yolk you want
- Chilled Drinkdrink temperature, fridge, freezer, or ice bath → minutes until cold
- Hot Caroutside temperature, minutes parked → cabin temperature
- Sunbathingskin type, UV index, SPF → minutes before burn
- Party Planningguest count, hours → food, drink, and ice quantities
- Christmas Treeroom height, topper, clearance, tree style → tree height, lights, ornaments
- Helium Balloonsballoon size, payload → balloons needed for lift
- Snowmantarget height → ball diameters, snow volume, build time
Thought experiments
5 toolsReal equations, playful premises. The output is a model, not a forecast.
- Lost Socksloads per week, socks per load → socks lost per year and the cost
- Dating Theorystart age, target age, partners per year → where the 37 percent stopping point lands
- Alien Civilizationthe seven Drake equation factors → communicating civilizations in the galaxy
- Vampire Apocalypsepopulation, bite rate → days until the last human
- Flat vs. Round Earthobserver height, distance → hidden height under each model
Six formulas, worked with real figures
Each card shows the equation the page runs, the inputs, and the number the page prints. Type the same inputs into the linked tool and the result should match to the cent. If your hand calculation disagrees, the rounding section below is the usual reason.
Mortgage
M = P·r / (1 - (1 + r)^-n)
- P = 250,000
- r = 6% / 12 = 0.005
- n = 30 × 12 = 360
- M = 1,250 / 0.83396
= 1,498.88 a month
Interest over the full term: 289,595.47. More than the house.
Run these numbers →Compound Interest
A = P(1 + r/k)^(k·t)
- P = 10,000, r = 5%, t = 10
- k = 1 (annual): 16,288.95
- k = 12 (monthly): 16,470.09
- k = 365 (daily): 16,486.65
= 181 from annual to monthly
Monthly to daily adds 16.56. The frequency dropdown matters once, then stops mattering.
Run these numbers →BMI
BMI = kg / m² or 703 × lb / in²
- 70 kg, 1.75 m
- 70 / 3.0625 = 22.86
- 154 lb, 69 in
- 703 × 154 / 4,761 = 22.74
= 22.9, inside 18.5 to 24.9
The 703 is the unit conversion folded into one constant. Skip the constant and imperial input reads 0.03.
Run these numbers →Percentage
change = (new - old) / old
- 80 to 92: 12 / 80 = +15.00%
- 92 to 80: -12 / 92 = -13.04%
= a 15% rise is a 13% fall
The denominator is always the starting value. Reversing the trip changes the denominator, so the percentages never match.
Run these numbers →Kinetic Energy
KE = ½ · m · v²
- m = 1,500 kg
- v = 60 mph = 26.82 m/s
- 0.5 × 1,500 × 719.4
= 539,581 J, about 540 kJ
At 30 mph the same car carries 134,895 J. Half the speed, a quarter of the energy. The square is the whole lesson.
Run these numbers →Exponential Decay
N(t) = N₀ · (½)^(t / t½)
- carbon-14, t½ = 5,730 years
- t = 10,000 years
- (½)^1.745 = 0.2983
= 29.8% remains
The decay constant form gives the same curve: k = ln 2 / 5,730 = 0.000121 per year. The tool accepts either.
Run these numbers →Five ways the input lies to the formula
A wrong result from a calculator is almost never a wrong formula. The formulas are the textbook ones. The result is wrong because a field got a number in the wrong unit, the wrong scale, or the wrong period, and the arithmetic carried on without complaint.
The rate is annual. The field is not always.
Loan, Mortgage, Compound Interest, and Investment all take a yearly rate and divide by the period count themselves. Type 0.5 because you already divided by 12 and the tool models a half-percent loan. Type 6 and let the page do the split.
Loan Calculator →Percent means the number before the sign.
Every percentage field on this site wants 15 for fifteen percent, not 0.15. A decimal in a percent box returns a result a hundred times too small, and the page has no way to tell a typo from a tiny rate.
Percentage Calculator →Term is a dropdown, and the two dropdowns do not meet.
The loan page offers 12 to 84 months. The mortgage page offers 15 to 30 years. An eight-year car loan or a ten-year home loan fits neither, and there is no free-text term box on either page. For those, pick the nearest term and read the payment as a bracket, or model the loan as a mortgage with a zero down payment and accept the 15-year floor.
Mortgage Calculator →Units are converted on the page, once.
BMI switches between kilograms with centimetres and pounds with feet and inches. Kinetic Energy accepts six mass units, from grains to tonnes, and six velocity units, from ft/s to km/s. Pick the unit from the selector next to the box. Typing "150 lb" into a kilogram field parses as 150 kilograms.
Kinetic Energy Calculator →Round at the end, not in the middle.
The pages keep full precision through the calculation and round only what they display. Reproduce a result by hand with a rounded monthly rate of 0.005 versus 0.004583 for 5.5% and the payment drifts by a few dollars across a 30-year term. When your figure and the page disagree by cents, check where you rounded.
Compound Interest Calculator →
What the result says, and what the formula left out
A formula answers the question in its own terms. BMI answers "weight divided by height squared" and nothing about health. A mortgage payment answers "principal and interest at this rate" and nothing about the escrow line. This table is the honest version of each group.
| Calculators | What the number is | What the formula leaves out |
|---|---|---|
| BMI, Ideal Weight, Body Fat | A screening band from height, weight, and tape measurements. | Muscle mass, bone density, age past 65, and pregnancy. A tape-measure body fat estimate lands a few points either side of a DEXA scan. A rugby forward reads as obese on BMI. |
| Loan, Mortgage | Principal and interest on a fixed rate. | Property tax, insurance, PMI, origination fees, and rate resets. The lender quote will be higher than the page. Use the page to compare rates and terms against each other, not to predict the bill. |
| Compound Interest, Investment, Retirement, Millionaire | Growth at one constant rate, pre-tax. | Market variance, fees, tax on gains, and inflation. Seven percent every year for thirty years is not how any real portfolio behaves. Treat the output as the center of a wide range. |
| Harris-Benedict BMR, Calorie | Resting and daily calorie estimates from a 1919 equation, revised in 1984. | Individual metabolism, which the equation misses by around 10 percent in either direction. Use the number as a starting point, weigh yourself for three weeks, and adjust. |
| Pregnancy | A due date 40 weeks after the last period. | Cycle length, ovulation timing, and ultrasound dating. Few babies arrive on the calculated date. A clinician will move the date after the first scan. |
| Sleep, Coffee Kick, Sunbathing | Timings from population averages: 90-minute sleep cycles, a five-hour caffeine half-life, skin-type burn times. | You. Sleep cycles run 70 to 120 minutes. Caffeine half-life spans two to twelve hours depending on genetics, pregnancy, and medication. Read these as a first guess. |
| Vampire Apocalypse, Alien Civilization, Lost Socks, Dating Theory | The equation, applied to the premise, with your numbers. | Any claim about the world. The Drake equation swings by orders of magnitude on one factor. These pages exist to show how a model responds to inputs, and they are honest about being models. |
Two-decimal output looks precise. The body calculators carry a ten percent error band, the kitchen calculators a few minutes either side, the thought experiments an order of magnitude. Read the display precision as a formatting choice, not a confidence interval.
Questions people bring after the result looks off
Why does the loan page show a different payment from my bank?
The page computes principal and interest only. A bank quote adds escrow for tax and insurance, sometimes PMI, and sometimes fees rolled into the principal. Compare the interest-and-principal line on the quote to the page figure. Those two should match to the cent when the rate and term agree.
Which compounding frequency should I pick?
Match the account. Savings accounts in most countries compound daily and pay monthly. Bonds compound semi-annually. If the paperwork does not say, monthly is the closest default. On 10,000 at 5 percent for ten years, the gap between monthly and daily is under 17.
Is my BMI wrong because I entered pounds and inches?
No, as long as the imperial toggle was on. The page multiplies by 703 to convert. A result near 0.03 means pounds and inches went into the metric fields. Switch the toggle and enter the figures again.
Do these calculators send my numbers anywhere?
No. Each page runs its formula in your browser with JavaScript. There is no account, no save, and no server call for the calculation. Reload the page and the fields reset.
How do I get a percent decrease from the percentage tool?
Use the change mode, put the larger number as the starting value and the smaller as the new value. The result carries a minus sign. Percent decrease from 92 to 80 is 13.04 percent, not 15, because the denominator is 92.
Is a novelty calculator result usable in a school assignment?
Paste the working, not the conclusion. Vampire Apocalypse is a compartment model from epidemiology. Alien Civilization is the Drake equation. Both show the formula and the intermediate steps, which is what an assignment grades. The premise is the part to leave out.
When this category is the wrong shelf
Amortization schedules with a full payment table, break-even, ROI, and tax figures live under Finance. Fractions, roots, matrices, and unit conversion sit under Math. Mean, standard deviation, and regression are in Statistics. Age in days, business days, and countdowns are in Date and Time. Each has its own guide.
