Percentages
X% of N = N × (X ÷ 100)
Examples:• 18% of 240 = 240 × 0.18 = 43.2
• 20% of $86 = $86 × 0.20 = $17.20
• 7.5% of $200 = $200 × 0.075 = $15.00
Mental math shortcut: For 10%, just move the decimal left one place. For 20%, double the 10% amount. For 5%, halve the 10% amount. For 15%, add 10% + 5%.
% change = ((New value − Old value) ÷ Old value) × 100
Examples:• Price went from $80 → $95: ((95 − 80) ÷ 80) × 100 = +18.75% increase
• Stock dropped from $150 → $120: ((120 − 150) ÷ 150) × 100 = −20% decrease
A positive result = increase. A negative result = decrease. The formula is always the same — the sign tells you the direction.
When you know the part and the percentage but need the whole:
43 ÷ 0.18 = 238.9 (approximately 239)
Example: You paid $12 which was a 15% tip — what was the bill?
$12 ÷ 0.15 = $80
Whole = Part ÷ (Percentage ÷ 100)
Example: 43 is 18% of what number?43 ÷ 0.18 = 238.9 (approximately 239)
Example: You paid $12 which was a 15% tip — what was the bill?
$12 ÷ 0.15 = $80
⚡ Percentage Calculator
All three percentage calculations in one place — find the percentage, the part, or the whole.
Tip & bill splitting
US tipping norms by service:
These are norms, not rules. Tip based on quality of service and your comfort level.
| Service | Standard tip | Exceptional |
|---|---|---|
| Sit-down restaurant | 18–20% | 25%+ |
| Bar / drinks | $1–2/drink or 15–20% | 20% |
| Food delivery | 15–20% | 20%+ |
| Taxi / rideshare | 15–20% | 20%+ |
| Hair salon | 15–20% | 25% |
| Hotel housekeeping | $2–5/night | $5–10/night |
| Coffee shop / counter | 10–15% (optional) | 15–20% |
| Buffet | 5–10% | 10–15% |
Per person = (Bill × (1 + tip%)) ÷ number of people
Example: $86.40 bill, 20% tip, 4 people:$86.40 × 1.20 = $103.68 total ÷ 4 = $25.92 per person
Mental math trick for 20%: Divide the bill by 10 (move decimal left), double it = 20% tip.
$86.40 → $8.64 × 2 = $17.28 tip → $103.68 total
For uneven splits or custom per-person items, use our tip and bill splitter.
Dates & time
Date arithmetic is trickier than it looks because months have different lengths and leap years add a day to February. The easiest way is to use a calculator — but here's the concept:
Convert both dates to a day number (days since a reference point), then subtract. Most programming languages, spreadsheets, and our date difference calculator do this automatically.
In a spreadsheet: Type the two dates in cells, then subtract them. Format the result cell as a number (not a date) to see the count of days.
Useful for: contract deadlines, pregnancy due dates, event countdowns, age in days, days since an event.
Convert both dates to a day number (days since a reference point), then subtract. Most programming languages, spreadsheets, and our date difference calculator do this automatically.
In a spreadsheet: Type the two dates in cells, then subtract them. Format the result cell as a number (not a date) to see the count of days.
Useful for: contract deadlines, pregnancy due dates, event countdowns, age in days, days since an event.
Your age in years = today's year minus your birth year, adjusted if your birthday hasn't happened yet this year.
Example: Born March 15, 1990. Today is October 13, 2026. Birthday has passed this year → 2026 − 1990 = 36 years old.
If today were February 1, 2026 — birthday hasn't happened yet → 2026 − 1990 − 1 = 35 years old.
Our age calculator also shows your age in months, weeks, and days — and can count days until your next birthday.
Example: Born March 15, 1990. Today is October 13, 2026. Birthday has passed this year → 2026 − 1990 = 36 years old.
If today were February 1, 2026 — birthday hasn't happened yet → 2026 − 1990 − 1 = 35 years old.
Our age calculator also shows your age in months, weeks, and days — and can count days until your next birthday.
Discounts & sales
Sale price = Original price × (1 − discount%)
Examples:• 30% off $80 = $80 × 0.70 = $56.00
• 15% off $129.99 = $129.99 × 0.85 = $110.49
Or to find the discount amount first:
Discount amount = $80 × 0.30 = $24 → Sale price = $80 − $24 = $56
Stacked discounts (e.g., 20% off + extra 10% off) are NOT additive. They're sequential:
$100 → 20% off = $80 → 10% off $80 = $72 (not $70)
Everyday calculators