Calculators
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Calculators is one page of twelve local calculators: percentage, date, age, tip, BMI, ovulation, salary, grade, pregnancy, inflation, mortgage, and loan. Math runs in the browser. The ?calc= query opens a type; numbers you type are not stored in that link.
Twelve local calculators in one page: percentage, date, age, tip, BMI, ovulation, salary/wage, grades, pregnancy, inflation, mortgage, and generic loans. Pick a type; the ?calc= query is shareable. No values are uploaded.
Each calculator section states its inputs, the formula the code uses, one numeric example, and what the result does not decide.
Inputs are two numbers and a mode. “Percent of” is (A / 100) × B, so 15% of 200 is 30. Other modes cover “A is what percent of B,” percent change, increase, decrease, and reversing a percent. The result is arithmetic only.
Inputs are calendar dates. The day count is the signed whole-day difference between local dates. From 1 January to 31 January is 30 days. An inclusive option adds one day when the span moves forward. Adding days uses the calendar, including month lengths.
Inputs are a birth date and a reference date. Age is whole years, months, and days on the local calendar. Days until the next birthday count forward to the next matching month and day. A retirement date adds a whole number of years from 18 to 120. The tropical zodiac label is a month-and-day table, not an astronomical position.
Inputs are a bill, an optional tax, a tip percent, a split count, and an optional service charge (percent or flat). Tip can be figured on the pre-tax or post-tax amount. A $50 pre-tax bill at 20% is a $10 tip; split by 2, each person’s tip share is $5 before service. Money is rounded to cents.
Metric BMI is weight in kilograms divided by height in meters squared. 70 kg at 1.75 m is 70 / (1.75 × 1.75) = 22.9. US customary is 703 × pounds / inches squared. Adult categories use cutoffs 18.5, 25, 30, 35, and 40. This is screening math, not a diagnosis, and it is not for ages under 20.
Inputs are the first day of the last period, a cycle length from 15 to 45 days, and a luteal length from 8 to 20 days. Ovulation is estimated cycleDays − lutealDays after that start. A 28-day cycle with a 14-day luteal phase points at day 14. The fertile window shown is the five days before ovulation through ovulation day. It is not a fertility diagnosis.
Inputs are a pay amount and a schedule. Defaults are 40 hours a week, 52 weeks a year, 26 biweekly periods, 12 months, and 260 workdays (5 × 52). $30 an hour at those defaults is 30 × 40 × 52 = $62,400 a year, $5,200 a month, and $1,200 a week. It is not a tax or payroll withholding calculation.
Grades share a 0–100 scale. A weighted average is sum(grade × weight) / sum(weight). A 90 with weight 60 and a 70 with weight 40 is (5400 + 2800) / 100 = 82. The final-exam mode solves for the final score that hits a target course grade. Weights are relative; only their ratio matters.
From the first day of the last menstrual period, the estimated due date is 280 days later. 1 January 2026 plus 280 days is 8 October 2026. From conception, the due date is 266 days later. An IVF day-3 transfer adds 263 days; a day-5 transfer adds 261 days. These are calendar conventions, not a clinical dating scan.
Constant-rate mode uses only your percent: future dollars = present × (1 + rate) ^ years. $100 at 3% for 10 years is $134.39. Year-span mode uses bundled U.S. CPI-U annual averages from 1913 through 2024: equivalent dollars = amount × (CPI target / CPI start). It is not a forecast and not a tax index.
Fixed monthly principal and interest is P × r × (1 + r) ^ n / ((1 + r) ^ n − 1), where r is the annual rate divided by 12 and n is the term in months. $200,000 at 6% for 360 months is $1,199.10 a month before taxes, insurance, or fees. A zero rate divides principal by the number of months. Extra payments shorten the schedule in the amortization view.
The loan calculator uses the same fixed-rate monthly payment formula as the mortgage tool, without home-price framing. You can solve for the payment, the principal that fits a maximum payment, or a comparison of rate and term. It ignores fees you do not type and is not a credit offer.
No. Percentage, date, age, tip, BMI, cycle, salary, grade, pregnancy, inflation, mortgage, and loan math run in your browser. Changing the dropdown updates the ?calc= query so you can bookmark a type.
Choose Mortgage calculator (?calc=mortgage) for home-loan payment and amortization views. Use Loan calculator for a generic fixed-rate loan that is not framed as a mortgage.
No. BMI is a simple screening formula. Ovulation and pregnancy tools are date estimates only. Confirm health decisions with a clinician.
Yes. Each type has a URL such as /tools/calculators?calc=tip. The numbers you type are not stored in that link unless you copy them yourself.
They use the inputs you provide (and published formulas in the UI). They are not a tax, payroll, or government inflation publication.
Open Percentage (?calc=percentage#calc-percentage). Percent of is (percent / 100) × the number. 15% of 200 is 30.
Open Date (?calc=date#calc-date). The default count is the whole-day difference. 1 January through 31 January is 30 days.
Open Age (?calc=age#calc-age). The result is years, months, and days from the birth date to the reference date on the local calendar.
Open Tip (?calc=tip#calc-tip). Tip equals the chosen base times the tip percent. That tip, any service charge, and the bill are divided by the number of people.
Open BMI (?calc=bmi#calc-bmi). Metric BMI is kilograms divided by meters squared. 70 kg at 1.75 m is 22.9. Adult category cutoffs start at 18.5, 25, 30, 35, and 40.
Open Ovulation (?calc=ovulation#calc-ovulation). Estimated ovulation is cycle length minus luteal length after the period start. A 28-day cycle with a 14-day luteal phase points at day 14. This is not medical advice.
Open Salary (?calc=salary#calc-salary). At the defaults, annual pay is hourly × 40 × 52. $30 an hour is $62,400 a year.
Open Grade (?calc=grade#calc-grade). Weighted grade is sum(grade × weight) / sum(weight). A 90 weighted 60 and a 70 weighted 40 equal 82.
Open Pregnancy (?calc=pregnancy#calc-pregnancy). From the last menstrual period the estimate adds 280 days. From conception it adds 266 days. It is not a diagnosis.
Open Inflation (?calc=inflation#calc-inflation). A constant rate compounds as present × (1 + rate) ^ years. $100 at 3% for 10 years is $134.39. Year-to-year mode uses CPI-U annual averages from 1913 through 2024.
Open Mortgage (?calc=mortgage#calc-mortgage). The fixed payment is P × r × (1 + r) ^ n / ((1 + r) ^ n − 1), with r = annual rate / 12. $200,000 at 6% for 30 years is $1,199.10 before taxes and insurance.
Open Loan (?calc=loan#calc-loan). It uses the same fixed-rate monthly formula as the mortgage calculator. It is a generic installment estimate, not a mortgage and not a credit decision.