Every formula, worked example, and free calculator for age, date, time, unit conversions, tip, sleep, calorie burn, pregnancy, GPA, driving time, and 100+ more daily calculations — all in one place. No guesswork, just answers.
Calculate your exact age, find out what day you were born, discover your star sign, and plan birthday milestones — down to the day.
Your exact age in years, months, and days is not as simple as dividing by 365. Leap years, varying month lengths, and the starting day all matter. The correct approach is to work backward from today's date in calendar steps:
Age in years = Current year - Birth year (adjusted if birthday not yet passed)
Age in months = Complete calendar months since last birthday
Age in days = Days elapsed since last monthly anniversary
Precise age in days = Days elapsed since birth date (accounting for leap years)
A year is a leap year if: it is divisible by 4, AND (not divisible by 100, OR divisible by 400). So 2000 was a leap year, 1900 was not, and 2024 was. If you were born on February 29, your official birthday in non-leap years is typically observed on February 28 or March 1 depending on the country.
| Milestone | Age (Years) | Legal/Cultural Significance |
|---|---|---|
| Voting age (US) | 18 | US constitutional right |
| Full legal adult (US) | 18 | Contracts, military, etc. |
| Alcohol (US) | 21 | Varies by country (18 in most) |
| Car rental (most agencies) | 25 | Lower insurance surcharges |
| Social Security (early US) | 62 | Reduced benefit |
| Full Social Security (US) | 67 | Born 1960 or later |
| Medicare (US) | 65 | Federal health program |
Want to know what day of the week you were born? The Doomsday algorithm by mathematician John Conway lets you calculate any date's day of the week mentally. For digital purposes, our Day of the Week Calculator handles it instantly for any date in history.
Days between dates, add or subtract days, countdown timers, time zone conversions, working days, and calendar week numbers.
To count exact days between two calendar dates, the most reliable method is to convert both dates to their Julian Day Number (JDN) and subtract. The Julian Day Number is an integer count of days since a fixed reference point (noon on January 1, 4713 BC). Most date calculators use this method internally.
Days = JDN(End date) - JDN(Start date)
Simple approximation: Days = (Y2-Y1)x365 + (M2-M1)x30 + (D2-D1)
Then correct for actual month lengths and leap years
Working days = Total days - weekends - public holidays
To add N days to a date: convert the start date to JDN, add N, convert back to calendar date. This automatically handles month boundaries and leap years. Common use cases: contract deadlines (add 30, 60, 90 days), warranty expiry dates, and project planning timelines.
Every time zone is defined by its offset from Coordinated Universal Time (UTC). To convert between zones: convert the source time to UTC first, then add the destination's UTC offset. During daylight saving time (DST), offsets shift by +1 hour for most regions in spring and back in fall.
Destination time = Source time + (Destination UTC offset - Source UTC offset)
Example: New York (UTC-5) to Tokyo (UTC+9): add 14 hours
If result exceeds 24h, add 1 day. If below 0h, subtract 1 day.
The ISO 8601 standard defines week 1 as the week containing the year's first Thursday. A year can have 52 or 53 weeks. Week numbers are widely used in European business, manufacturing, and project management. Week 1 of 2026 began on December 29, 2025.
| Calculation | Formula / Method | Common Use |
|---|---|---|
| Days until an event | Event date - Today's date | Countdowns, deadlines |
| Add business days | Skip Saturdays, Sundays, and holidays | Contract terms, SLAs |
| Quarter of the year | Quarter = ceil(Month / 3) | Business reporting |
| Day of year (ordinal) | Sum of days in prior months + day | Julian dates, science |
| Unix timestamp | Seconds since Jan 1, 1970 00:00:00 UTC | Software, databases |
Instantly convert between measurement systems — metric and imperial — for length, weight, temperature, volume, speed, and more.
1 inch = 2.54 cm exactly (defined by international agreement)
1 foot = 30.48 cm | 1 yard = 0.9144 m | 1 mile = 1.60934 km
1 meter = 3.28084 feet | 1 km = 0.621371 miles
1 nautical mile = 1.852 km (used in aviation and maritime)
1 pound (lb) = 0.453592 kg | 1 kg = 2.20462 lbs
1 ounce (oz) = 28.3495 grams | 1 stone = 14 lbs = 6.35029 kg
1 ton (US short) = 2,000 lbs | 1 tonne (metric) = 1,000 kg = 2,204.62 lbs
Fahrenheit to Celsius: C = (F - 32) x 5/9
Celsius to Fahrenheit: F = (C x 9/5) + 32
Celsius to Kelvin: K = C + 273.15
Kelvin to Celsius: C = K - 273.15
| Temperature | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | -273.15 | -459.67 | 0 |
| Water freezing | 0 | 32 | 273.15 |
| Room temperature | 20-22 | 68-72 | 293-295 |
| Body temperature | 37 | 98.6 | 310.15 |
| Water boiling | 100 | 212 | 373.15 |
Volume conversion is critical in cooking, chemistry, and construction. The US and imperial (UK) systems both use cups, pints, quarts, and gallons — but the values differ. 1 US gallon = 3.785 liters. 1 UK gallon = 4.546 liters. Always check which system a recipe or specification uses.
Calculate the perfect tip, split restaurant bills fairly, handle unequal splits, and figure out tip on pre-tax vs post-tax amounts.
Tip amount = Bill total x (Tip % / 100)
Total bill = Bill + Tip amount
Per person (even split) = Total bill / Number of people
Per person (weighted split) = (Person's items / Total) x Bill + (Tip share)
| Service | Standard Tip | Excellent Service |
|---|---|---|
| Restaurant (sit-down) | 15-18% | 20-25% |
| Restaurant (buffet) | 10% | 15% |
| Bartender | $1-2 per drink | 20%+ of tab |
| Taxi / Rideshare | 10-15% | 20% |
| Food delivery | 15-20% | 20-25% |
| Hair salon | 15-20% | 25% |
| Hotel housekeeping | $2-5 per night | $5-10 per night |
| Spa / Massage | 15-20% | 20-25% |
For 20%: Move the decimal point one place left (that is 10%), then double it. On a $47.50 bill: 10% = $4.75, doubled = $9.50 tip. For 15%: Find 10% then add half. $47.50: 10% = $4.75, half = $2.38, total tip = $7.13. For 25%: Find 10%, double it, add half of the original 10%.
Find the best bedtime or wake-up time based on 90-minute sleep cycles. Wake up refreshed, not groggy, every morning.
Human sleep is not a single continuous state. You cycle through four distinct stages approximately every 90 minutes: NREM Stage 1 (light sleep, 5-10 min), NREM Stage 2 (true sleep, 20 min), NREM Stage 3 (deep sleep, 20-40 min), and REM sleep (dream stage, 10-60 min). Waking mid-cycle — especially during deep sleep or REM — causes sleep inertia, that groggy disoriented feeling.
Ideal bedtime = Wake time - (Cycles x 90 min) - 15 min (time to fall asleep)
Ideal wake time = Bedtime + 15 min + (Cycles x 90 min)
Recommended cycles: 5-6 per night (7.5 - 9 hours of sleep)
Minimum: 4 cycles (6 hours) — not recommended long-term
| Age Group | Recommended Sleep | Ideal Cycles |
|---|---|---|
| Newborn (0-3 months) | 14-17 hours | 9-11 |
| Infant (4-11 months) | 12-15 hours | 8-10 |
| Toddler (1-2 years) | 11-14 hours | 7-9 |
| School-age (6-13 years) | 9-11 hours | 6-7 |
| Teenager (14-17 years) | 8-10 hours | 5-6 |
| Adult (18-64 years) | 7-9 hours | 5-6 |
| Older adult (65+) | 7-8 hours | 5 |
Calculate calories burned during exercise, steps to calories, walking and running calorie burn, and daily activity totals.
The standard method for calculating calories burned during physical activity uses MET (Metabolic Equivalent of Task) values. A MET of 1.0 represents the energy you burn at rest (sitting quietly). Running at 6 mph has a MET of 9.8, meaning you burn 9.8 times more energy than at rest.
Calories burned = MET x Weight (kg) x Duration (hours)
Example: 80 kg person running (MET 9.8) for 30 min:
9.8 x 80 x 0.5 = 392 calories
Steps to calories: Calories = Steps x 0.04 (approximate, varies by weight/stride)
| Activity | MET Value | Cal/hr (70 kg person) |
|---|---|---|
| Sitting (resting) | 1.0 | 70 |
| Walking (3 mph) | 3.5 | 245 |
| Walking (fast, 4 mph) | 5.0 | 350 |
| Cycling (moderate) | 6.8 | 476 |
| Running (6 mph) | 9.8 | 686 |
| Running (8 mph) | 11.8 | 826 |
| Swimming (freestyle) | 8.3 | 581 |
| Jump rope | 11.8 | 826 |
Due date calculation, conception date, how many weeks pregnant, fetal age, and baby growth milestone calculators.
The most widely used due date formula was developed by German obstetrician Franz Karl Naegele in 1812. Despite its age, it remains the standard used by doctors worldwide. The average human pregnancy is 280 days (40 weeks) from the last menstrual period.
From LMP: Due date = LMP date + 280 days
Naegele's Rule: LMP date + 1 year - 3 months + 7 days
From conception: Due date = Conception date + 266 days (38 weeks)
Gestational age (weeks) = (Today - LMP date) / 7
| Trimester | Weeks | Key Developments |
|---|---|---|
| First trimester | Weeks 1-12 | Organ formation, most miscarriage risk |
| Second trimester | Weeks 13-26 | Movement felt, anatomy scan at 20 weeks |
| Third trimester | Weeks 27-40 | Rapid growth, lung maturation |
| Full term | Weeks 39-40 | Optimal delivery window |
| Post-term | Week 42+ | Induction typically recommended |
Reading time by word count, words to pages, typing speed, readability scores, and manuscript length estimators.
Reading time (min) = Word count / Reading speed (WPM)
Average adult reading speed: 200-300 WPM (non-fiction)
Technical reading: 100-150 WPM | Speed reading: 500-1000 WPM
Pages to words: Approx. 250-300 words per double-spaced page
Words per minute (spoken): ~130 WPM average for presentations
| Content Type | Length | Reading Time (250 WPM) |
|---|---|---|
| Blog post (short) | 500 words | 2 minutes |
| Blog post (long) | 2,000 words | 8 minutes |
| Short story | 7,500 words | 30 minutes |
| Novella | 40,000 words | 2.7 hours |
| Novel (average) | 90,000 words | 6 hours |
| Novel (long) | 150,000 words | 10 hours |
Drive time, distance, pace calculators, time zone travel planning, and road trip time estimation.
Drive time = Distance / Average speed
Total trip time = Drive time + Stops + Traffic buffer (10-20%)
Distance = Speed x Time
Average speed = Total distance / Total time
| Road Type | Average Speed | Notes |
|---|---|---|
| Urban streets | 20-30 mph | Includes stops and lights |
| Suburban roads | 30-45 mph | Mix of lights and open road |
| Rural highways | 50-60 mph | Speed limit typically 55 mph |
| Interstate / freeway | 60-70 mph | Speed limit 65-75 mph |
| Long road trip (mixed) | 50-55 mph | Accounting for all conditions |
Love compatibility, anniversary countdowns, numerology, soulmate calculators, and other fun life tools.
Random number generators, name pickers, coin flip, dice rollers, and decision-making tools for every day.
Screen time, grade calculators, retirement countdowns, height predictors, and other everyday utility tools.
All formulas and reference data on this guide are sourced from current authoritative standards and institutions: