Guides
100 concise how-to articles that explain the formulas behind our tools. Jump by topic below, or open any calculator from thehome page.
Percentage Calculator
Open tool →How to Calculate Percentage Increase (With Examples)
Learn the percentage increase formula, work through clear examples, and avoid common mistakes when comparing old and new values.
Read guide →How to Calculate Percentage of a Number (With Examples)
Step-by-step formula for finding X% of Y, reverse percentages, discounts, and tips — with worked dollar and score examples.
Read guide →Percentage Decrease and Discount Math Explained
Formulas for percentage decrease and sale prices, with worked retail examples and common stacking mistakes to avoid.
Read guide →Find the Original Price Before a Discount (Reverse Percentage)
Work backwards from a sale price and discount percent to recover the original price — with formulas and retail examples.
Read guide →What Is 20 Percent of a Number? (Worked Examples)
Find 20% of any number with the simple multiply-by-0.2 rule, plus 10%, 15%, and 25% shortcuts and money examples.
Read guide →Percent Off vs Percent of vs Percentage Difference
Tell discount “percent off,” “percent of,” and percentage difference apart — with formulas and retail examples.
Read guide →Compound Percentage Increase Over Two Steps
How to chain two percentage increases (or an increase then a decrease) correctly — multiply factors, do not add percents.
Read guide →Markup vs Margin Percentage (Clear Difference)
Markup and margin are not the same percent: formulas, conversion between them, and worked retail examples.
Read guide →How to Calculate Percent Change (Decrease)
Percent decrease formula from old to new value, worked price and metric examples, and how it differs from percent off.
Read guide →Double a Number by Percent (What +100% Means)
How doubling relates to percent: +100% doubles, 200% of a number, and common mix-ups with “200% increase.”
Read guide →Reverse Percentage: Find the Whole from a Part
How to reverse a percentage: if 18% is $45, what is 100%? Formulas, tax-included prices, and common reverse-percent mistakes.
Read guide →Successive Discounts vs One Combined Percent
Why 20% off then 10% off is not 30% off: multiply remaining factors, worked retail examples, and how to combine stacked discounts correctly.
Read guide →What Percent Is One Number of Another?
Formula for “X is what percent of Y?”, worked score and budget examples, and how this differs from percent change.
Read guide →Increase by a Percent Then Decrease Back
Why +20% then −20% does not return to the start: multiply factors, worked examples, and how to reverse a percent change correctly.
Read guide →Split Percentages That Add to 100
Allocate shares that must total 100%: normalizing uneven parts, rounding so the pie closes, and worked budget examples.
Read guide →Percent Left After a Decrease
After a 25% decrease, what percent remains? Convert “off” to “left,” with retail and score examples and reverse checks.
Read guide →Half Then Double With Percentages
What −50% then +100% does to a number, why order matters with percents, and clear factor examples that avoid common mix-ups.
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Tip Calculator
Open tool →How Much to Tip: Percentages, Bill Splits, and Examples
A practical guide to tip percentages in the US, how to tip on tax, and how to split a restaurant bill fairly.
Read guide →How to Calculate Tip Without a Calculator
Mental math shortcuts for 10%, 15%, 18%, and 20% tips, plus how to split a bill fairly at the table.
Read guide →How to Split a Bill Evenly With Tip
Step-by-step equal bill splits including tip: formulas, per-person examples, and when unequal shares make more sense.
Read guide →Gratuity vs Tip vs Service Charge: What Differs
How tip, gratuity, and service charge differ on restaurant bills, when each appears, and how to calculate what you actually pay.
Read guide →How to Tip 15% vs 18% vs 20% (Side-by-Side Examples)
Compare 15%, 18%, and 20% tips on the same bill amounts, with per-person splits and when each rate is common.
Read guide →Tip Calculator for Large Groups (Split Fairly)
How to tip and split a large restaurant bill: auto-gratuity, per-person math, tax handling, and fair splits for big parties.
Read guide →How to Split Tip by Meal Cost Share
Split a restaurant tip proportionally to what each person ordered — formulas, worked examples, and when equal splits are simpler.
Read guide →Restaurant Tip Etiquette Basics (US)
Practical US restaurant tipping basics: common percent ranges, tax and auto-gratuity, takeout vs table service — descriptive, not preachy.
Read guide →Sales Tax vs Tip Percentage (Do Not Mix Them Up)
Sales tax and tip use different bases and purposes — formulas, US restaurant examples, and how to avoid stacking mistakes.
Read guide →How Much to Tip for Takeout
US takeout tipping patterns in plain numbers: common percent ranges, counter prompts, delivery vs pickup, with worked examples.
Read guide →Tipping on Discounted Bills
How people tip when a coupon or discount lowers the check: tip on pre-discount, post-discount, or auto-gratuity — with worked examples.
Read guide →Tip Before or After Tax?
Should you tip on the pre-tax subtotal or the after-tax total? Custom norms, worked dollar examples, and how sales tax differs from tip percent.
Read guide →How to Tip for Delivery Orders
Practical delivery tipping: percent vs flat amounts, bad weather and distance, app prompts vs cash, and worked examples for food delivery.
Read guide →Rounding Tips to Whole Dollars
How people round tip amounts up or down to whole dollars, when to round after percent math, and fair split examples.
Read guide →Tipping When a Service Charge Is Included
What to do when the bill already lists a service charge or auto-gratuity: avoid double tipping by mistake, with worked check examples.
Read guide →How to Tip Rideshare Drivers
Practical rideshare tipping: percent vs flat amounts, short vs long trips, airport runs, and how app prompts differ from cash.
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Unit Converter
Open tool →Metric ↔ Imperial Unit Conversion: Length, Weight, Temp, Volume
Quick reference and worked examples for converting cm/inches, kg/lb, °C/°F, and litres/gallons without memorizing every factor.
Read guide →Celsius to Fahrenheit Explained (Formula + Examples)
Clear °C ↔ °F formulas, memory anchors (0 °C = 32 °F, 100 °C = 212 °F), and everyday weather and cooking examples.
Read guide →Kg to Lbs and Cm to Inches: Clear Walkthroughs
Exact factors and worked examples for kilograms ↔ pounds and centimetres ↔ inches — luggage, height, and DIY.
Read guide →Miles to Km and Pounds to Kilograms (Tables + Examples)
Conversion factors, quick reference tables, and worked examples for miles ↔ kilometres and pounds ↔ kilograms.
Read guide →Cups to mL and Ounces to Grams (Conversion Only)
Neutral US cup ↔ millilitre and ounce ↔ gram conversion factors and tables — measurement math, not recipes.
Read guide →Temperature Conversion Tricks Without a Calculator
Mental math shortcuts for °C ↔ °F: double-and-add-30, exact formulas when you need them, and common weather and oven anchors.
Read guide →Fahrenheit to Celsius Quick Formula
The exact °F → °C formula, a fast mental estimate, worked weather and oven examples, and common sign mistakes.
Read guide →Boiling Water: Celsius vs Fahrenheit
Boiling point of water at sea level in °C and °F, altitude notes, and quick conversion back to everyday temperatures.
Read guide →Oven Temperature C to F (Common Baking Settings)
Convert common oven temperatures from Celsius to Fahrenheit with the exact formula, a quick table, and rounding notes for recipes.
Read guide →Body Temperature C vs F (Unit Conversion Only)
Convert body-temperature style readings between °C and °F with the standard formula. Unit math only — not medical advice or diagnosis.
Read guide →Feet and Inches to Centimeters (Height & DIY)
Convert feet and inches to centimeters with exact factors, height examples, and the reverse path from cm back to ft/in.
Read guide →Liters to Gallons Quick Conversion
Convert litres to US and UK gallons with exact factors, fuel and bottle examples, and the reverse gallons-to-litres path.
Read guide →Meters to Feet and Inches
Convert metres to feet and inches with exact factors, height and room examples, and the reverse path back to metres.
Read guide →Fahrenheit Weather to Celsius Quick Chart
Everyday weather anchors from °F to °C (freezing, cool, warm, hot) with the exact formula — unit conversion only, not health advice.
Read guide →Yards to Meters Quick Conversion
Convert yards to metres with the exact factor, sports-field examples, and the reverse metres-to-yards path.
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Date Difference Calculator
Open tool →How to Calculate Days Between Two Dates
Find the number of days, weeks, or approximate months between dates, and how to add N days to a start date.
Read guide →How Many Days Until a Date (Countdown + Business Days)
Count days until an event, understand inclusive counting, and estimate business days between two calendar dates.
Read guide →How to Add Days to a Date (Date Plus N Days)
Add or subtract N calendar days from a start date for deadlines, warranties, and schedules — with leap-year examples.
Read guide →Business Days Between Dates (Exclude Weekends)
Count weekdays between two dates, skip weekends, and estimate working days for deadlines — with step-by-step examples.
Read guide →How Many Weeks Between Two Dates
Convert days between dates into weeks and leftover days, with inclusive counting notes and deadline examples.
Read guide →How Many Months Between Two Dates
Count calendar months between dates, approximate month spans from days, and avoid inclusive-counting mistakes.
Read guide →Business Days vs Calendar Days
Difference between business days and calendar days for deadlines: weekends, simple counting examples, and related date tools.
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Age Calculator
Open tool →How to Calculate Age from Date of Birth
Compute exact years, months, and days from a birthday — including as-of a chosen date — with leap-year notes.
Read guide →Exact Age in Days, Weeks, and Months
How to express age as total days, weeks, or calendar months — and when years/months/days is the right answer instead.
Read guide →Leap Year Age Calculation Pitfalls
How leap days affect age and date math: Feb 29 birthdays, year spans that include Feb 29, and common off-by-one mistakes.
Read guide →Chronological Age vs Completed Years
What “years completed” means versus calendar chronological age, how forms differ, and worked birthday examples.
Read guide →Turning 40 in Leap Years (Birthday Math)
How leap days affect “turning 40” and other milestone birthdays — calendar age, Feb 29 cases, and as-of date examples.
Read guide →Calculate Age on a Future Date
Find how old someone will be on a future as-of date — completed years, Y/M/D, and planning examples for cutoffs and milestones.
Read guide →How Old Was Someone on a Past Date?
Calculate completed age on a past as-of date for records, stories, and eligibility checks — with Y/M/D examples and leap-day notes.
Read guide →Age Eligibility Cutoffs Explained
How age cutoffs work for leagues, schools, and tickets: as-of dates, birthday rules, and how to check eligibility with an age calculator.
Read guide →How to Calculate Age in Total Days Only
Count exact age as a single day total between birth date and an as-of date — formulas, leap days, and when day-counts beat Y/M/D.
Read guide →School-Year Age by September Cutoff
How September (or similar) age cutoffs assign school years: as-of dates, birthday edges, and checking eligibility with an age calculator.
Read guide →Age in Completed Months Only
Count age as completed months for baby and toddler charts: month-boundary rules, worked examples, and when days still matter.
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Word Counter
Open tool →Word Count for Essays and Reading Time Estimates
Typical essay word counts, how reading time is estimated (~200 wpm), and when character limits matter more than words.
Read guide →Character Count vs Word Count: When Each Matters
Compare character limits (with/without spaces) to word counts for SMS, bios, essays, and SEO — with concrete examples.
Read guide →How Long to Read 1000 Words (Reading Time Guide)
Estimate reading time for 1000 words and other lengths using ~200–250 wpm, plus speaking-time tips for presentations.
Read guide →Average Reading Speed in Words Per Minute
Typical silent and oral reading speeds (wpm), how sites estimate “min read,” and what changes the number.
Read guide →How Many Characters in a Tweet-Style Limit?
Count characters vs words for short posts and bio limits: spaces, punctuation, and how a word counter helps you stay under a cap.
Read guide →Reading Time From Word Count Formula
Estimate reading time from word count using words-per-minute assumptions, with examples for 500–3000 words and when to adjust speed.
Read guide →Speaking Time From Word Count
Estimate how long a script takes to speak from word count using words-per-minute assumptions, with examples and pacing tips.
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Case Converter
Open tool →Title Case vs Sentence Case (When to Use Each)
Compare title case and sentence case for headlines, emails, and UI copy — plus snake_case and kebab-case for code and URLs.
Read guide →Snake_case vs Kebab-case: When to Use Each
When to choose snake_case for code identifiers and kebab-case for URLs or CSS — with conversion examples.
Read guide →camelCase vs PascalCase vs snake_case
When to use camelCase, PascalCase, and snake_case in code and APIs — with side-by-side conversion examples.
Read guide →When to Use Title Case in Headlines
When title case fits headlines, emails, and UI labels versus sentence case — with clear before/after examples.
Read guide →All Caps to Title Case Cleanly
Turn YELLING headlines into readable Title Case: what to capitalize, small-word exceptions, and how case converters help without overdoing it.
Read guide →Kebab Case for URL Slugs
Why kebab-case fits URL slugs: readability, separators vs snake_case, and practical rules for generating clean paths.
Read guide →Sentence Case for Buttons and Labels
Why UI buttons and form labels often use sentence case: readability, consistency with Title Case headlines, and conversion tips.
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Password Strength Meter
Open tool →How to Check Password Strength Safely (Local Only)
Why password checks should stay in your browser, what length and character variety mean, and common weak patterns to avoid.
Read guide →Strong Password Checklist (and Common Mistakes)
A practical strong-password checklist: length, uniqueness, managers, and the most common password mistakes that still get accounts breached.
Read guide →How to Create a Memorable Strong Password
Build a memorable yet strong password with passphrase methods, length over clever substitutions, and when a password manager is still better.
Read guide →Passphrase vs Complex Password: Which Is Stronger?
Compare passphrases and complex short passwords: length vs character rules, memorability, and when a password manager still wins.
Read guide →Password Manager vs Remembering Passwords
When a password manager beats memorized passwords, what to still memorize, and how uniqueness and length fit together.
Read guide →How Long Should a Password Be?
Practical password length guidance: why length beats clever substitutions, common site minimums, and passphrases vs random strings.
Read guide →Why Unique Passwords Matter Per Site
Why reusing one password across sites multiplies breach risk, how uniqueness pairs with length, and practical ways to keep unique logins.
Read guide →Should You Change Passwords After a Breach?
When to change a password after a breach notice, what to change first, and how uniqueness and a password manager limit the blast radius.
Read guide →Password Expiration Policies: Pros and Cons
Why forced periodic password changes fell out of favor, when rotation still helps after a suspected leak, and better habits than calendar expiry.
Read guide →Uppercase vs Lowercase in Passwords
How letter case affects password strength and login matching, why a single capital is a weak rule, and better length-first habits.
Read guide →Password vs Passcode vs PIN
How passwords, passcodes, and PINs differ in length and use, when a short numeric PIN is enough, and how uniqueness still matters.
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Random List Picker
Open tool →How to Pick Randomly Without Bias
Fair random picks from a list: equal odds, why “first thought” and bad shuffles bias results, and how a random list picker helps.
Read guide →Randomize a List for a Giveaway Fairly
Run a fair giveaway draw from a list: one entry per person, equal odds, transparent process, and biases to avoid.
Read guide →Making a Fair Raffle With a List
Run a fair raffle from a name or ticket list: equal odds, duplicates, multiple prizes, and transparent draw steps.
Read guide →Seeded Shuffle Explained Simply
What a seeded shuffle means in plain language: same seed → same order, why it helps reproducible raffles and tests, and fairness caveats.
Read guide →Drawing Without Replacement From a List
How to pick multiple winners or order a list fairly without putting names back: equal odds, duplicates, and step-by-step draw logic.
Read guide →How to Pick a Random Order for Speakers
Fair speaking or presentation orders from a name list: shuffle once, draw without replacement, and optional seeded reproducibility.
Read guide →Removing Duplicates Before a Fair Draw
Clean a name list before a raffle or random pick: when to dedupe, when duplicates are tickets, and how cleanup affects fairness.
Read guide →Fair Team Split From a List
Split a name list into fair teams: shuffle then deal, balance sizes, avoid captain bias, and optional seeded reproducibility.
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