Practical guides, how-tos and comparisons to get the most out of our free online tools.
Gestational age is counted from your last period, about two weeks before conception, so a pregnancy of 6 weeks contains an embryo of about 4 weeks. Here is Naegele's rule derived, the correction for a cycle that is not 28 days, and when ACOG says an early scan should replace your dates.
Sofia Nunes · 13 min read
A Unix timestamp counts the seconds since January 1, 1970 UTC. Learn why computers use it, how to convert it, and the 2038 problem.
Daniel Okonkwo · 2 min read
Daylight saving time shifts clocks an hour in spring and back in autumn. Learn spring-forward and fall-back, why one day is 23 hours and another 25, and how US and EU dates differ.
Lena Hoffmann · 4 min read
An employee costs far more than their gross salary. Learn how payroll taxes, benefits and overhead push the total to roughly 1.25–1.4× the salary, with a worked example.
Daniel Okonkwo · 4 min read
Find the slope of a line from two points with m = (y₂ − y₁) / (x₂ − x₁), and read what a positive, negative, zero, or undefined slope tells you.
Lena Hoffmann · 4 min read
PPI and DPI both measure density per inch, but one is about screens and the other about print. Learn how to compute pixel density and what Retina really means.
Daniel Okonkwo · 5 min read
Watts per kilogram is the number that decides who climbs fastest. Learn how W/kg works, what it means for FTP and how to improve it.
Aisha Karim · 3 min read
Learn how to estimate your maximum heart rate, use the Karvonen formula and train in the right zones so every session hits its intended intensity.
Aisha Karim · 3 min read
How to split your daily calories across meals and snacks, with typical per-meal ranges and how to adjust them to your goals.
Sofia Nunes · 2 min read
Work out how many concrete blocks a wall needs from its area, add mortar and a waste allowance, and avoid a second trip to the yard.
Marco Bianchi · 3 min read
Turn an hourly rate into a yearly salary: multiply rate by hours per week by weeks worked. See the full-time math, gross vs. net, and worked examples.
Camille Laurent · 3 min read
Converting between time zones is adding or subtracting the difference in UTC offsets, then handling any day rollover. Learn the method, a worked example, and the DST caveat.
Lena Hoffmann · 5 min read