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Finance, health, math, science, unit conversion, business, food, sports, education, and daily planning. Every calculator includes the tool, formula notes, examples, FAQs, and related guides.

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Five calculator collections

Every calculator is organised into one of five topic areas. Click a collection to browse all the tools, sub-categories, and guides it contains.

Finance

137 calculators

Loans, mortgages, savings, investments, tax, budgets, retirement planning and more.

Finance
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Health & Fitness

385 calculators

BMI, calories, nutrition, body composition, fitness training, sports and wellbeing.

HealthSportsFood
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Science & Engineering

755 calculators

Physics, chemistry, construction, unit conversion, technology and technical design.

ConstructionConversionPhysicsChemistryEngineeringTechnology
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Business & Data

252 calculators

Profit margins, ROI, pricing, statistics, payroll, revenue and business planning.

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Everyday Life

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Math, dates, discounts, tips, education, ecology and everyday household decisions.

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Calculator categories

Each category page works like a focused mini-library with topic clusters, descriptions, internal links, and calculator pages targeting specific search intent.

Finance

Plan loans, savings, investments, taxes, budgets, interest, and everyday money decisions with clear formulas and examples.

137 calculators

Health

Estimate health, fitness, nutrition, body measurements, and wellness metrics using practical reference formulas.

135 calculators

Math

Solve percentages, ratios, fractions, averages, geometry, algebra helpers, and common school or workplace math problems.

131 calculators

Construction

Estimate materials, areas, volumes, costs, paint, concrete, flooring, roofing, and renovation quantities.

130 calculators

Conversion

Convert units for length, weight, temperature, speed, volume, area, pressure, energy, and daily measurements.

125 calculators

Physics

Work with speed, force, energy, motion, electricity, waves, pressure, and classroom science formulas.

125 calculators

Chemistry

Use chemistry helpers for concentration, molarity, dilution, gas laws, molecular mass, and lab calculations.

125 calculators

Statistics

Analyze averages, probability, samples, confidence, variation, scores, and data interpretation problems.

125 calculators

Business

Estimate profit, margins, markup, inventory, payroll, pricing, revenue, productivity, and business planning metrics.

127 calculators

Everyday Life

Quick calculators for dates, time, tips, discounts, travel, fuel, recipes, shopping, and household planning.

110 calculators

Sports

Calculate pace, performance, training zones, scores, ratios, sports statistics, and fitness planning numbers.

125 calculators

Food

Scale recipes, estimate calories, cooking conversions, ingredient weights, nutrition, and meal planning quantities.

125 calculators

Ecology

Estimate energy use, water use, carbon footprint, recycling, transport impact, and sustainable living metrics.

125 calculators

Engineering

Work through mechanical, electrical, civil, and systems engineering formulas for practical design checks.

125 calculators

Education

Support learning with grade, score, GPA, reading, study time, classroom math, and academic planning calculators.

125 calculators

Technology

Estimate bandwidth, storage, download time, screen sizes, data transfer, battery life, and computing metrics.

125 calculators

Accuracy and transparency

Why United Calculator

Every page follows the same content standard: formula derivation, a worked example with real numbers, applicable laws or professional guidance, 6 plain-language FAQs, and links to 8 related tools.

Verified formulas

Each formula engine is based on published standards — TILA for loan calculations, METS tables for energy expenditure, ASTM standards for materials, and equivalents across 16 subject areas. The formula is shown in a code block on every result card so you can verify the calculation independently.

Professional context, not professional advice

Finance and health calculators cover applicable legislation (CFPB, NHS, OSHA) and reference professional thresholds (e.g. debt-to-income ratios accepted by lenders, WHO BMI categories). Every page includes a disclaimer — these tools are planning aids. Consequential decisions should be reviewed with a qualified professional.

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Transparent methodology

Smart defaults come from published benchmarks — median UK household income from ONS, US average mortgage rate from Freddie Mac weekly surveys, average commute distances from the US Census ACS. Defaults are documented in the article body on each calculator page.

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How United Calculator works

Three steps from question to answer — no account, no subscription, no ads cluttering the result.

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Type a topic into the search box or click a category. Each category is a mini-library of 125 calculator pages targeting specific questions — loan payments, BMI, tile counts, carbon footprint, and more.

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Caffeine Half-Life Calculator

See when caffeine clears your system using the 5-hour half-life formula. Includes drink presets (espresso, filter coffee, tea), a decay chart, and sleep impact warning.

The caffeine half-life calculator models how caffeine clears from your body over time, using pharmacokinetic data showing an average half-life of 5 hours in healthy adults. Half-life means the time for plasma concentration to halve — so 200mg at hour 0 becomes ~100mg at hour 5, ~50mg at hour 10, and ~25mg at hour 15. The "safe to sleep" threshold in this calculator is set at 50mg, below which most people experience minimal stimulant effect. and are related tools for tracking energy and recovery.

Half-life varies significantly between individuals: smokers metabolise caffeine roughly twice as fast (half-life ~3h); pregnant women in the third trimester metabolise it three times more slowly (half-life ~15h); people taking certain medications (notably oral contraceptives and some antibiotics) also have extended half-lives. The 5-hour average used here is a population midpoint.

  1. Select a drink preset (espresso, filter coffee, energy drink, tea) or enter a custom mg dose.
  2. Enter the time you consumed the caffeine using the hour and minute fields.
  3. Read the chart — each point shows estimated caffeine remaining in mg at that hour.
  4. Check the "safe to sleep" line (50mg) — your bedtime should be after your caffeine level crosses below this line.
  5. If you drink multiple cups, add the doses together or run the calculator twice and sum the values at your target bedtime.

The caffeine half-life formula explained

Caffeine remaining (mg) = Dose × 0.5^(hours elapsed ÷ 5)

This is a standard first-order pharmacokinetic decay model. The exponent (hours ÷ 5) represents elapsed time as a fraction of the half-life. Raising 0.5 to this power gives the proportion of the original dose remaining.

Worked example: 200mg at 08:00, half-life 5h → at 13:00 (5h later): 200 × 0.5^1 = 100mg. At 18:00 (10h): 200 × 0.5^2 = 50mg. At 21:00 (13h): 200 × 0.5^2.6 ≈ 33mg. Cut-off to sleep at 23:00: 200 × 0.5^3 = 25mg — below the 50mg threshold, so sleep should be minimally affected.

Reading your caffeine half-life results

What does the 50mg sleep threshold mean?

Sleep researchers generally use 50mg as the approximate threshold below which adenosine receptors are no longer meaningfully blocked and sleep onset is not significantly delayed. Above 50mg at bedtime, sleep latency (time to fall asleep) increases, slow-wave deep sleep duration decreases, and overall sleep efficiency drops — even if you can still fall asleep. The threshold varies individually: caffeine-sensitive individuals may be affected by as little as 20mg; habitual high-dose consumers develop tolerance and may tolerate 100mg+ without measurable sleep impact.

The chart's dashed 50mg line gives you an immediate visual of when your caffeine level crosses into the safe zone. Use the alongside this to time your last cup relative to your planned bedtime.

Health tips and best practices

Common mistakes to avoid

Caffeine pharmacokinetics data comes from peer-reviewed pharmacology literature. The 5-hour half-life is a population average and may differ significantly for individuals based on genetics (CYP1A2 enzyme variants), medications, liver function, pregnancy, and smoking status. This tool is for general information only — not medical advice. If you are pregnant, breastfeeding, or taking medication, consult a healthcare provider about safe caffeine limits. The NHS recommends no more than 200mg/day during pregnancy.