Glossary

Forty terms behind the calculators, defined in plain English — each linked to the tool that puts it to work

Every definition here matches how the term is actually used inside the linked calculator — no circular dictionary-speak. Curated and verified by Andrius R. · Updated June 2026.

💰 Finance

APR (Annual Percentage Rate)

The yearly cost of borrowing including mandatory fees, not just the interest rate — the one number designed to make loan offers comparable. Two loans with identical rates can have different APRs. Used throughout the loan calculator.

APY / AER (Annual Percentage Yield)

The effective annual return once compounding frequency is included: a nominal 6% compounded monthly is really 6.17% APY. Banks quote APY when paying you and nominal rates when charging you — convert both before comparing. See the interest calculator.

Amortization

Paying a loan down with fixed payments that each split between interest (on the remaining balance) and principal. Early payments are mostly interest; the split flips over time — the amortization calculator shows the full month-by-month table.

Principal

The amount actually borrowed or invested, before any interest. "Paying down principal" is the only part of a payment that reduces the debt.

Compound interest

Interest calculated on the current balance including past interest — growth on growth. The reason $10,000 at 7% becomes $76,000 in 30 years instead of simple interest's $31,000. The engine of the compound interest calculator.

Equity

The portion of an asset you truly own: market value minus what's still owed on it. Home equity grows from payments, extra payments and price appreciation.

Inflation

The general rise of prices over time, equivalently the fall of money's buying power — at 3% a year, prices double roughly every 24 years. Measured by indexes like CPI. Quantify it with the inflation calculator.

Real vs nominal

Nominal values are in the money of their day; real values are adjusted for inflation. A 5% raise during 7% inflation is a real-terms pay cut. Long-term projections in the investment calculator should always state which one they're using.

Marginal vs effective tax rate

The marginal rate is what your next dollar of income pays; the effective rate is total tax ÷ total income. A US single filer on $60,000 sits in the 12% bracket but pays about 8.4% overall — see the worked example in the income tax calculator.

Credit utilization

Card balances as a percentage of credit limits — roughly 30% of a credit score, and the fastest factor to improve. Stay under ~30%; the best scorers sit in single digits. Explained in the credit score estimator.

Snowball / avalanche

The two classic debt payoff orders: snowball clears the smallest balance first (fast wins), avalanche the highest interest rate first (least total interest). Compare both on your own debts with the debt payoff calculator.

VAT (Value-Added Tax)

A consumption tax collected in slices along the supply chain and ultimately paid by the final consumer. To remove VAT from a gross price, divide by (1 + rate) — never subtract the percentage. The VAT calculator handles both directions.

ROI (Return on Investment)

Gain divided by cost, as a percentage. Useful but easily abused — it ignores time, so a 25% ROI over 3 years (≈7.7%/year) and over 10 years are very different deals. The ROI calculator adds the annualized view.

🏥 Health

BMR (Basal Metabolic Rate)

The calories your body burns at complete rest — typically 60–70% of your daily total. Estimated here with the Mifflin-St Jeor equation (accurate to roughly ±10%). The foundation of the BMR calculator.

TDEE (Total Daily Energy Expenditure)

Your real daily burn: BMR scaled by an activity multiplier (×1.2 sedentary to ×1.9 very active). The number that eating decisions should actually be based on — computed by the calorie calculator.

BMI (Body Mass Index)

Weight (kg) divided by height (m) squared — a population screening tool that can't tell muscle from fat and says nothing about where fat sits. Useful as a flag, not a verdict; the BMI calculator covers its limits honestly.

Body fat percentage

The share of your weight that is fat rather than muscle, bone and water — the question BMI dodges. Estimated by tape measurements (US Navy method, ±3–4%) in the body fat calculator.

Macronutrients (macros)

Protein and carbohydrate (4 kcal per gram) and fat (9 kcal per gram) — the three buckets every calorie target splits into. Protein gets set first, by body weight; see the macro calculator.

Calorie deficit / surplus

Eating below (deficit) or above (surplus) your TDEE. A 500 kcal/day deficit targets roughly 1 lb (0.45 kg) of fat loss per week — a planning approximation that overestimates over long periods as the body adapts.

Heart rate zones

Training intensity bands as percentages of maximum heart rate (Zone 2, 60–70%, is the endurance workhorse). Max HR is estimated (Tanaka: 208 − 0.7 × age) with ±10 bpm of real individual spread. Mapped in the heart rate calculator.

Sleep cycle

A ~90-minute pass through light, deep and REM sleep; adults need the equivalent of 5–6 per night. Alarms landing mid-cycle cause sleep inertia — the sleep calculator does the backwards arithmetic.

Gestational age

Pregnancy duration counted from the last menstrual period — about two weeks before conception — written as weeks+days (e.g. 32+4). Due date = LMP + 280 days, an anchor only ~4–5% of babies hit exactly. See the pregnancy calculator.

🔢 Math

Mean, median, mode

Three "averages": the mean (sum ÷ count) is dragged by outliers; the median (middle value) resists them — which is why incomes are reported as medians; the mode (most frequent) suits categories like shoe sizes. Compared in the average calculator.

Standard deviation

The typical distance of values from their mean — the trust rating on an average. For bell-shaped data, ~68% of values fall within one SD and ~95% within two. Sample SD divides by n−1; population SD by n.

Percentage points vs percent

A rate moving from 4% to 6% rose 2 percentage points but 50 percent. News reports mix these constantly; the difference is enormous. Untangled in the percentage calculator.

GCD (Greatest Common Divisor)

The largest number dividing evenly into all of yours — found in two steps by Euclid's 2,300-year-old algorithm. The tool for simplifying fractions and ratios; see the LCM & GCD calculator.

LCM (Least Common Multiple)

The smallest number all of yours divide into — the common denominator for adding fractions and the meeting point of repeating schedules. For two numbers, LCM × GCD = their product.

Prime factorization

Breaking a number into its unique product of primes (360 = 2³ × 3² × 5) — the "DNA" that reveals every divisor and powers RSA encryption. Done step-by-step in the prime factorization calculator.

Independent events

Events whose outcomes don't affect each other — their combined probability multiplies (two sixes: 1/6 × 1/6 = 1/36). Believing past coin flips change future ones is the gambler's fallacy. See the probability calculator.

Ratio & proportion

A ratio compares quantities (flour to sugar, 2:3); a proportion states two ratios are equal, solved by cross-multiplication. Note 1:4 squash-to-water means 1 part in 5 total — 20%, not 25%. The ratio calculator handles scaling and splitting.

Degrees vs radians

Two units for angles: 180° = π radians. The classic calculator trap — sin(30) is 0.5 in degree mode and −0.988 in radian mode. Check the mode light before any trig; see the scientific calculator guide.

🔬 Science

Mass vs weight

Mass (kg) is how much matter you are — the same everywhere. Weight (newtons) is gravity's force on that mass: W = m × g, so your 70 kg weighs 687 N on Earth but 113 N on the Moon. Untangled in the force calculator.

Newton (N)

The SI unit of force: what accelerates 1 kg by 1 m/s². For intuition, roughly the weight of a small apple resting in your palm.

Kinetic energy

Energy of motion: KE = ½mv². The v² is the important part — doubling speed quadruples the energy, and the braking distance with it. Worked through in the energy calculator.

Kilowatt-hour (kWh)

The electricity bill's unit: 1,000 watts running for one hour (3.6 million joules). Energy = power × time — a 2 kW heater for 30 minutes is 1 kWh. Watts measure the rate; kWh measure the amount.

Ohm's law

V = I × R: voltage (electrical pressure) equals current (flow) times resistance (pipe narrowness). With P = V × I it explains fuses, cable thickness and why US kettles are weak. See the Ohm's law calculator.

Density

Mass per unit volume (ρ = m ÷ V) — the material fingerprint that identified Archimedes' fake crown. Objects float when their average density is below the fluid's; water's 1,000 kg/m³ is the yardstick. See the density calculator.

Absolute zero / Kelvin

−273.15 °C, the floor of temperature where molecular motion is minimal. The Kelvin scale starts there, which makes it the only scale where "twice the temperature" means twice the thermal energy. See the temperature converter.

SI units

The international measurement system (meter, kilogram, second, ampere, kelvin…) modern definitions tie to physical constants — the meter is defined from the speed of light. Conversion factors like 1 inch = 2.54 cm are exact by definition; see the unit converter.

Atomic number

The count of protons in an atom's nucleus — the element's identity. Change it and you have a different element; it's the number that orders the periodic table.

📅 Date & Time

Leap year

A 366-day year that keeps the calendar synced with Earth's 365.2425-day orbit. The rule has stacked exceptions: divisible by 4, except centuries, except those divisible by 400 — so 2000 leaped and 2100 won't. Handled automatically in the age calculator.

Inclusive vs exclusive counting

Monday to Friday is 4 days by subtraction (exclusive) or 5 by counting both ends (inclusive) — the "fence-post problem" behind most date off-by-one errors. Both modes explained in the days between dates calculator.

Business days

Weekdays excluding weekends (and usually public holidays): a 30-day month holds about 22 of them. Contracts and shipping quotes specify business or calendar days deliberately — they differ by ~40%.

UTC (Coordinated Universal Time)

The atomic-clock world reference all time zones are offsets from (Lithuania: UTC+2/+3). The reliable way to convert between zones is via UTC, never directly — see the time zone converter.

Daylight Saving Time (DST)

The seasonal clock shift that different regions apply on different dates (and the Southern Hemisphere applies in reverse) — which is why the New York–Vilnius gap is 7 hours most of the year but 6 for a few weeks each spring.

Gregorian calendar

The 1582 calendar reform in worldwide use today, adopted at different times by different countries (Britain: 1752, deleting 11 days). Software extends its rules backwards ("proleptic"), so historical dates may differ from contemporary documents — caveats noted in the day of week calculator.