What Is a Healthy Waist-to-Height Ratio?
Keeping your waist under half your height is a simple health marker. Learn what waist-to-height ratio means, how to measure, and why under 0.5 is the goal.
Sofia Nunes · 3 min read
Guides & tutorials
Practical guides, how-tos and comparisons to get the most out of our free online tools.
February 10, 2026
February 9, 2026
How to Use the Quadratic Formula: Solving ax² + bx + c = 0
Solve any quadratic equation with x = (−b ± √(b² − 4ac)) / 2a, and read the discriminant to know how many real solutions to expect.
Lena Hoffmann · 5 min read
How to Compare Unit Prices and Find the Better Deal
Turn any package price into a price per pound or ounce so you can compare sizes and brands honestly, ignore the marketing, and spot the real bargain on the shelf.
Marco Bianchi · 4 min read
How Much Roofing Material Do I Need? Area and Squares
Turn your building footprint into true roof area using the roof pitch, then convert to roofing squares or square meters plus a waste allowance.
Marco Bianchi · 3 min read
February 6, 2026
Splitting Rent Between Rooms That Are Not the Same
Equal thirds of $2,400 charge the same for 180 square feet and for 100. Floor area is better and still leaves someone envious. The sealed-bid envy-free split gives $850, $700 and $850 — and nobody wants to swap.
Camille Laurent · 18 min read
How Far Can I Drive on the Fuel I Have Left?
Estimate remaining range from fuel left and your car's economy. A simple method for gas and diesel, with a worked example and a safety margin.
Marco Bianchi · 3 min read
How to Convert kWh to CO₂: Grid Emission Factors Explained
Convert electricity in kWh into CO₂ emissions using your grid's emission factor — and understand why that factor differs so much by country and time.
Marco Bianchi · 3 min read
February 5, 2026
Adjustable-Rate Mortgages and the Caps That Bound Them
A 5.00 percent start with 2/2/5 caps can legally reach 10.00 percent and a payment of $2,484.14 — 54.3 percent above where it began. That worst case is computable before you sign, and it is the only number that should decide the choice.
Camille Laurent · 17 min read
How Much Rent Can I Afford? The 30% Rule and 40x Rent
Set a realistic rent budget with the 30% rule and the 40x-rent test, and learn why using net income instead of gross keeps you on safer ground.
Camille Laurent · 3 min read
February 4, 2026
What Is a Break-Even ROAS? (And How to Find Yours)
ROAS is revenue divided by ad spend. Learn what break-even ROAS means, how to find it from your profit margin, and why a ROAS above 1 can still lose money.
Daniel Okonkwo · 4 min read
What Is Voltage Drop? Why Long Wires Lose Voltage
Voltage drop is the voltage lost along a wire due to its resistance. Learn how length, gauge, and current affect it, and why it matters for your circuits.
Lena Hoffmann · 4 min read
February 3, 2026
Interest-Only Mortgages: What You Are Deferring
Ten interest-only years on a $300,000 loan at 6 percent cost $180,000 and repay nothing. When amortisation starts the payment jumps from $1,500.00 to $2,149.29 — 43.3 percent overnight — and the loan ends up $48,315.79 dearer than the repayment version.
Camille Laurent · 16 min read