How to Calculate — step-by-step guides

Buying a home starts with one number: the amount you actually have to borrow. The total home price (including buyer's costs) minus your savings gives your mortgage need.

1. 1Calculator A (Total home price): the price of the home plus buyer's costs, for example €375,000.
2. 2Calculator B (Savings / down payment): the money you bring yourself, for example €75,000.
3. 3Calculator C (Mortgage need): A − B = €300,000. This is the amount you need to borrow.

Property ROI shows what a rental property earns per year as a percentage of everything you put in. Add the purchase price and extra costs to get your total investment, work out the net yearly rental income, and divide one by the other.

1. 1Calculator A (Purchase price): the price of the property, for example €300,000.
2. 2Calculator B (Extra costs): notary fees, renovation and other one-off costs, for example €25,000.
3. 3Calculator C (Total investment): A + B = €325,000 — everything you put in.
4. 4Calculator D (Monthly rent): the rent you receive, for example €1,400.
5. 5Calculator E (Yearly rent): D × 12 = €16,800.
6. 6Calculator F (Yearly costs): maintenance, taxes and empty months, for example €2,800.
7. 7Calculator G (Net yearly income): E − F = €14,000.
8. 8Calculator H (Property ROI): G ÷ C × 100 = 4.31% — your yearly return on the total investment.

Start with the original price and your discount percentage. Calculate the amount saved in a separate calculator, then subtract it to see the final price. Changing either input updates both results.

1. 1Calculator A (Original price): enter €250, displayed in euros.
2. 2Calculator B (Discount percentage): enter 20, displayed as 20%.
3. 3Calculator C (Discount amount): A × B% = €50. The % key turns 20 into 0.20 for this calculation.
4. 4Calculator D (Final price): A − C = €200. Change the price or the percentage and both amounts update instantly.

Compound interest means you earn interest on your interest: every year the rate is applied to the new, larger amount. This template chains one calculator per year, so you see the growth step by step — change the starting amount or the rate and every year recalculates.

1. 1Calculator A (Starting amount): your initial savings, for example €10,000.
2. 2Calculator B (Yearly rate): the interest rate, for example 5%.
3. 3Calculator C (Growth multiplier): 1 + B% = 1.05. Each year the balance is multiplied by this factor.
4. 4Calculators D–H (Year 1 to Year 5): each year multiplies the previous year by C. After 5 years: €12,762.82.
5. 5Calculator I (Interest earned): Year 5 − A = €2,762.82 — the total interest on top of your own money.

A wedding budget grows with every guest. Multiply your guest count by the catering price per guest, add the venue and the other big costs, then keep a 10% buffer for surprises. Change the number of guests and the whole budget recalculates.

1. 1Calculator A (Guest count): the number of guests, for example 80.
2. 2Calculator B (Food & drinks per guest): the catering price per person, for example €110.
3. 3Calculator C (Total catering): A × B = €8,800.
4. 4Calculator D (Venue rental): for example €3,500.
5. 5Calculator E (Attire, rings & styling): for example €2,600.
6. 6Calculator F (Photo & video): for example €2,200.
7. 7Calculator G (Music, flowers & stationery): for example €1,900.
8. 8Calculator H (Subtotal wedding costs): C + D + E + F + G = €19,000.
9. 9Calculator I (Buffer rate): 10% for unexpected costs.
10. 10Calculator J (Contingency buffer): H × I% = €1,900.
11. 11Calculator K (Total wedding budget): H + J = €20,900.
12. 12Calculator L (Cost per guest): K ÷ A = €261.25 per guest.

Profit is what remains of your revenue after costs; the profit margin shows that profit as a percentage of revenue. Link the steps so every change flows through.

1. 1Calculator A (Revenue): your total sales, for example €12,500.
2. 2Calculator B (Costs): everything you spent to earn that revenue, for example €7,400.
3. 3Calculator C (Profit): A − B = €5,100.
4. 4Calculator D (Profit margin): C ÷ A × 100 = 40.8%.

When you write a quote or invoice, you start from your net price and add VAT on top. This template links your net price and VAT rate to the VAT amount and the final price your customer pays.

1. 1Calculator A (Price excl. VAT): your net price, for example €100.
2. 2Calculator B (VAT rate): the VAT percentage, for example 21% (or 12% or 6%).
3. 3Calculator C (VAT amount): A × B% = €21 — the VAT added on top.
4. 4Calculator D (Price incl. VAT): A + C = €121 — what your customer pays.

A receipt or gross price already contains VAT. To find the net price you divide by (1 + VAT rate) — simply subtracting 21% gives the wrong answer. This template does it correctly and shows how much VAT was included.

1. 1Calculator A (Price incl. VAT): the gross price, for example €121.
2. 2Calculator B (VAT rate): the VAT percentage, for example 21% (or 12% or 6%).
3. 3Calculator C (Price excl. VAT): A ÷ (1 + B%) = €100 — the net price.
4. 4Calculator D (VAT included): A − C = €21 — the VAT inside the gross price.

FIRE stands for Financial Independence, Retire Early. The popular 4% rule says you can safely withdraw about 4% of your portfolio each year. Divide your yearly expenses by 4% to get your FIRE number — the portfolio you need. Subtract what you already have, then divide the gap by what you save each year to see how many years until you are financially independent.

1. 1Calculator A (Yearly expenses): what you spend per year, for example €30,000.
2. 2Calculator B (Safe withdrawal rate): the 4% rule, so 4%.
3. 3Calculator C (FIRE number): A ÷ B% = €750,000 — the portfolio you need.
4. 4Calculator D (Current savings): what you have invested today, for example €150,000.
5. 5Calculator E (Amount still needed): C − D = €600,000.
6. 6Calculator F (Monthly savings): what you invest each month, for example €2,000.
7. 7Calculator G (Years to FIRE): E ÷ (F × 12) = 25 years (without counting investment returns).

BMI (Body Mass Index) is your weight in kilograms divided by your height in meters squared. WHO categories: under 18.5 underweight, 18.5–24.9 healthy weight, 25–29.9 overweight, 30 or higher obesity. Because the height squared is already calculated, you also see the healthy weight range for your height.

1. 1Calculator A (Weight): your weight in kg, for example 75.
2. 2Calculator B (Height): your height in cm, for example 180.
3. 3Calculator C (Height in meters): B ÷ 100 = 1.80 m.
4. 4Calculator D (Height squared): C × C = 3.24 m².
5. 5Calculator E (BMI score): A ÷ D = 23.15 — healthy weight (18.5–24.9).
6. 6Calculator F (Min. healthy weight): 18.5 × D = 59.94 kg.
7. 7Calculator G (Max. healthy weight): 24.9 × D = 80.68 kg.

Your TDEE (Total Daily Energy Expenditure) is how many calories you burn per day. It starts with your BMR, calculated with the Mifflin-St Jeor formula (men: +5, women: −161), multiplied by an activity factor. Subtract a calorie deficit — 500 kcal per day is about 0.5 kg per week — to get your daily calorie target.

1. 1Calculator A (Body weight): your weight in kg, for example 80.
2. 2Calculator B (Height): your height in cm, for example 180.
3. 3Calculator C (Age): your age in years, for example 30.
4. 4Calculator D (BMR): (10 × A) + (6.25 × B) − (5 × C) + 5 = 1,780 kcal/day. For women, use −161 instead of +5.
5. 5Calculator E (Activity factor): 1.2 sedentary, 1.375 lightly active, 1.55 moderately active, 1.725 very active. Example: 1.375.
6. 6Calculator F (TDEE): D × E = 2,447.50 kcal/day — your maintenance calories.
7. 7Calculator G (Calorie deficit): for example 500 kcal/day (about 0.5 kg per week).
8. 8Calculator H (Daily calorie target): F − G = 1,947.50 kcal/day.

How many days, hours or seconds have you been alive? Enter your birth year, month and day. The template counts the days between your birth date and today (opened today, using an average of 365.25 days per year and 30.44 days per month), then converts those days into every other unit.

1. 1Calculator A (Birth year): for example 1996.
2. 2Calculator B (Birth month): 1–12, for example 6 for June.
3. 3Calculator C (Birth day): 1–31, for example 15.
4. 4Calculator D (Age in years): D ÷ 365.25 — with D the age in days from Calculator G.
5. 5Calculator E (Age in months): D × 12.
6. 6Calculator F (Age in weeks): D ÷ 7.
7. 7Calculator G (Age in days): (this year − A) × 365.25 + (this month − B) × 30.44 + (today − C). Today's date is filled in when you open the template.
8. 8Calculator H (Age in hours): G × 24.
9. 9Calculator I (Age in minutes): H × 60.
10. 10Calculator J (Age in seconds): I × 60 — most people pass one billion seconds around age 31.7.

Guides for calculating your GPA, ROI on investments and freelance net income are on the way. In the meantime, you can build any of them yourself — see Tips & how it works.