CalcKit📚 Calculation Guides
Plain-English, step-by-step explainers for the questions behind the numbers. Each guide pairs with a free CalcKit calculator. New guides added regularly.
📈 Rule of 72: How Long to Double Your Money?
Divide 72 by your annual interest rate and you get roughly how many years it takes for money to double. The formula both ways, worked examples, accuracy limits and the inflation version.
⏰ How Is Overtime Calculated in Qatar?
Extra hours are paid above your normal rate: the legal 25% day premium, 50% night premium, Friday work rules, the basic-salary ÷ 240 hourly formula and a worked example.
⚡ BMR vs TDEE: What's the Difference?
BMR is what you burn at rest; TDEE adds movement, digestion and exercise. The difference explained, with the Mifflin-St Jeor formula, activity multipliers and a worked example.
🧬 How to Calculate Your Body Fat Percentage at Home
A tape measure is enough to estimate body fat: the US Navy method explained — what to measure, the formula, a worked example, and accuracy limits.
🏖️ How Much Should You Have Saved for Retirement by Age?
The 1x-by-30 to 10x-by-67 salary milestones explained: what they assume, a worked example, and what to do if you're behind — with a free calculator.
🤰 How to Calculate Your Pregnancy Due Date From Your Last Period?
Naegele's rule explained: the LMP + 280 days formula, the quick add-7-days/−3-months shortcut, adjustments for longer or shorter cycles, and how accurate a due date really is.
🏠 How Much House Can I Afford on My Salary?
The 28/36 rule lenders use, a quick 3–5× salary shortcut, a worked example, and the hidden costs that change the answer — with a free calculator.
🎓 How to Convert Percentage to GPA
A percentage and a GPA measure the same thing on different scales — but there is no single universal formula. The ÷9.5 rule, 4.0-scale bands, and how to pick the right one.
🏘️ How Much of Your Salary Should Go to Rent?
The classic 30% rent rule explained: how to calculate it on gross vs take-home pay, what counts as housing cost, and when it's okay to spend more — with a worked example.
💳 Debt Snowball vs Avalanche: Which Pays Off Faster?
Snowball (smallest balance first) vs avalanche (highest rate first): which method clears debt faster, which saves more interest, and how to pick — with a worked example.
⏱️ How to Calculate Your Hourly Rate From Salary
Convert any salary into an hourly rate with the 2,080-hour formula, adjusted for leave, holidays and overtime — plus the freelancer billable-hours version, with worked examples.
🔥 How Much Calorie Deficit to Lose 1kg a Week?
One kg of body fat stores about 7,700 kcal — here is the deficit formula that follows, a worked example, how to split it between diet and exercise, and safe ranges.
😴 How Much Sleep Do You Need by Age?
Newborns need 14–17 hours, adults 7–9. The recommended sleep hours for every age group, why they change, and how to turn them into a bedtime.
💵 How Much Should You Tip at a Restaurant in the US?
The 18–20% rule for US restaurants, when to tip more or less, delivery and bar tipping, and a quick mental-math trick for the tip.
💧 How to Calculate Daily Water Intake by Body Weight
Stop guessing with the 8-glasses rule. Here is how to calculate your daily water intake from your body weight — the 30–35 ml/kg formula, exercise adjustments, and a worked example.
📋 How to Calculate Gratuity in the UAE
End-of-service gratuity is money your employer legally owes you when you leave a job in the UAE. Here is the exact formula, who qualifies, and a worked example you can follow.
🕌 How to Calculate Zakat Nisab
Zakat becomes obligatory only once your wealth crosses the nisab threshold. Here is how to work out the nisab at today’s prices and check whether you owe zakat.
🏦 How EMI Prepayment Saves You Interest
Every extra payment toward your loan does double duty: it shrinks what you owe and wipes out all the future interest on that amount. Here is the math.
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