What Are Pack-Years?
A pack-year sums up a smoking history in one number. Here is how it is calculated and why doctors use it to judge risk and screening.
Sofia Nunes · 2 min read
Guides & tutorials
Practical guides, how-tos and comparisons to get the most out of our free online tools.
March 9, 2026
March 6, 2026
Sleeping Bag Temperature Ratings, and What Comfort Actually Means
The numbers on a sleeping bag come from ISO 23537-1, which publishes four figures and lets retailers print one. The standard measures a heated manikin's heat loss — so it measures insulation, and its own test mat is R 4.8. Put the same bag on an R 1.5 pad and the heat leaving through your back more than triples.
Aisha Karim · 14 min read
How Much Electricity Do Appliances Use? Watts, kWh and Cost
How watts turn into kilowatt-hours, what typical appliances draw, and how to translate that consumption into a cost on your bill.
Marco Bianchi · 3 min read
How Many Wall Studs Do I Need? Framing by Spacing and Length
Count the studs a wall needs from its length and stud spacing, then add for corners, openings and end studs so nothing is missing on framing day.
Marco Bianchi · 3 min read
What Is Body Surface Area (BSA)?
Body surface area estimates your body's external area in square metres. Here is how it is calculated and why clinicians use it to dose medicines.
Sofia Nunes · 2 min read
March 5, 2026
A Balloon Loan's Payment Is Small Because the Debt Stays
A balloon loan is priced on a long amortisation and repaid on a short one, and the whole instrument lives in that gap. On $250,000 at 6.5%, amortised over 30 years and due at 7, the payment is $1,580.17 and the lump sum still owed is $226,040.61 — 90.4% of what you borrowed.
Camille Laurent · 12 min read
How Much Food to Carry: The Number That Matters Is Calories per Gram
A published walking-energetics equation turns body mass, pack weight, speed and terrain into a daily calorie figure. But calories are not what you carry — mass is. Work the arithmetic and the planning number stops being calories per day and becomes calories per ounce.
Aisha Karim · 14 min read
Training Zones Are a Model, Not a Measurement
Every zone scheme is a set of percentages of some anchor, and the anchor is the whole argument. One runner, three published schemes: the ceiling of easy aerobic work lands at 131, 141 or 150 bpm. And if the anchor is 220 minus age, a two-standard-deviation error moves one boundary by 30 bpm.
Aisha Karim · 17 min read
March 4, 2026
Fiscal Horsepower Is Not Horsepower
Several European countries tax cars on an administrative power figure that nobody has ever measured — it is the output of a formula, the formula differs by country, and it has been rewritten more than once. Here is what each market actually taxes, worked through on one car.
Marco Bianchi · 15 min read
Degrees to Radians: The Conversion Explained
Convert between degrees and radians with radians = degrees × π/180 (and back with × 180/π), understand why 180° = π, and learn the common angles by heart.
Lena Hoffmann · 3 min read
March 3, 2026
Headlight Aim and How Far You Can Actually See
A dipped beam is aimed downward by a fraction of a degree, so its reach on flat road is a single division. Do it and you find that at motorway speed the beam is shorter than the stopping distance — and that a loaded boot can throw the whole calculation away.
Marco Bianchi · 12 min read
How to Calculate Billable Hours, Utilization and Annual Revenue
A step-by-step guide to billable hours: separate billable from non-billable time, work out your utilization rate, and project the annual revenue your hours can produce.
Daniel Okonkwo · 5 min read