207 guides on everyday. Showing the 48 most recent.
The arithmetic of an ordinary day: percentages, dates and durations, quantities, conversions. Short guides that show the working rather than just the answer, so the next one is a sum you do yourself.
Ranked by how many pages of this site link to them, not by traffic.
Life path, expression, soul urge, personality, birthday, maturity, personal year. Everything comes from a day, a month, a year and a name — and two of the seven are arithmetic restatements of the others, which is worth knowing before reading them as seven separate findings.
Antonio Delgado · 7 min read
The soul urge adds only the vowels, so the entire calculation turns on whether Y is one. Mary Taylor reads 6, 22 or 8 depending on the rule you apply, and every name in a 1,200-name corpus that contains a Y changes when you flip the setting.
Antonio Delgado · 6 min read
The two letter charts in circulation give the same name the same number only 10.3 % of the time, barely more than the one-in-nine you would get by drawing lots. Albert Einstein is a 9 in one chart and a 1 in the other, and neither reading is a mistake.
Antonio Delgado · 6 min read
The same 12.7 kg puppy at sixteen weeks predicts a 21.9 kg adult if it is a small breed and 38.5 kg if it is a giant one. The arithmetic is trivial; the class you pick is worth seventy-six per cent.
Lena Hoffmann · 6 min read
Two cats need three boxes, not two, and each should be one and a half body lengths long — 69 cm for an average adult. The litter figure that follows is larger than most people expect, and knowing why is the useful part.
Marco Bianchi · 6 min read
Length A + half B, width twice C, height D. The width rule is the one that fails people: a dog with 28 cm shoulders needs 56 cm of interior width, and no crate sold by weight rating tells you that.
Marco Bianchi · 6 min read
Add the digits of the whole date, or reduce day, month and year first and then add: two textbook methods for the same life path. Over the 46,021 dates from 1900 to 2025 they give different answers 6,398 times — 13.90 %. Every single disagreement is about a master number.
Antonio Delgado · 7 min read
Roll a 50 mm cone 80 mm tall and its pattern is a 190.8° slice of a 94 mm circle. Flatten a nearly cylindrical lampshade and the pattern radius runs to 839 mm — the shallower the taper, the further away the apex.
Lena Hoffmann · 6 min read
Thirty thousand stitches at 3.5 mm with fifteen per cent waste is 120.7 metres. The stitch count comes from the file; the average length is an assumption, and doubling it doubles the answer.
Antonio Delgado · 6 min read
At 1 %, a hundred attempts give you 63.4 % — not certainty. Ninety per cent takes 230 attempts and ninety-nine takes 459, and more than a third of players are still empty-handed at a hundred.
Lena Hoffmann · 4 min read
Venus never gets more than 47.2° from the Sun, so it is always within two signs of your Sun sign. Over 10,248 dates from 1966 to 2026 it lands in the same sign as the Sun barely a quarter of the time.
Antonio Delgado · 4 min read
A pattern asks for length and a shop sells weight. 400 yards of DK is 182 g and two balls; 400 yards of fingering is 100 g and one. The bridge is a single number on the label, and reading it wrong is the reason projects run out.
Antonio Delgado · 5 min read
The American numbers count up as the needle gets fatter and the British ones count down. They cross exactly once, at 4.5 mm, where both read 7 — and that single coincidence is the trap.
Antonio Delgado · 5 min read
Two stitches per 10 cm is the difference between a 50 cm scarf and a 55 cm one. The pattern's cast-on number is only correct for the person who wrote it.
Antonio Delgado · 6 min read
The same 100 × 120 chart finishes 23 cm wide on 11-count and 11.5 cm wide on 22-count — twice the width, four times the area, from one number on the fabric label.
Antonio Delgado · 5 min read
Points over total is trivial. What is not: averaging percentages instead of weighting them can shift a mark by twelve points, negative marking changes when guessing is worth it, and the same 62 % is a fail on one scale and an upper second on another. What the tool does, and the two things it cannot do.
Lena Hoffmann · 13 min read
Nine absences give five different rates depending on what you divide by — scheduled days, sessions, weekdays, calendar days, or a part-timer's own diary. What the tool computes, the three places it misleads, and why the Bradford Factor makes short repeated absences look ten times worse than one long one.
Camille Laurent · 11 min read
Twenty-five and five come from Francesco Cirillo's method, not from a trial. What the research actually measured is task switching and the cost of getting back in — and the reason a fixed interval helps is not that 25 minutes is optimal. Plus what the planner really returns, including the long break it quietly leaves out.
Lena Hoffmann · 12 min read
A calculator gives you an order of magnitude, and anything holding up a floor, a roof or people needs an engineer and a code check. Between those two facts there is a great deal worth knowing: which power of the span the deflection really follows, which loading case the tool assumes, and what L/360 is in fractions of an inch on a span you can measure.
Marco Bianchi · 13 min read
Knot strength is quoted as a percentage of the line's rated breaking strain, and the percentages in circulation come from tests nobody agrees on. What the calculator on this page really stores, what one peer-reviewed measurement actually found, where a line breaks and why, and how to match a knot to its job.
Antonio Delgado · 13 min read
Counting back and counting forward are two different sums, and they disagree by a year on the day itself. What this calculator does with 29 February, what other date libraries do, and where the traditional and modern gift lists actually come from.
Antonio Delgado · 11 min read
0800 is not “eight o'clock” and not “08:00” — it is “zero eight hundred”, with a zone letter on the end. The four digits, the spoken form, midnight at 0000, the argument about 2400, and the reason 12-hour notation cannot settle noon.
Daniel Okonkwo · 11 min read
Magnus–Tetens with the Alduchov–Eskridge coefficients, 17.625 and 243.04. Why an absolute number beats relative humidity for judging comfort, what temperature a window has to reach before it runs, and the one input the calculator does not check.
Lena Hoffmann · 11 min read
The calculator runs the 2001 index, not the 1945 one, and refuses to answer above 50 °F or under 3 mph. Here is what its number is a model of — bare skin, in shade, walking — and the two things it says nothing about: a clothed body, and a parked car.
Lena Hoffmann · 12 min read
Area grows with the square of the diameter, so an 18-inch pizza is not half as big again as a 12-inch — it is more than twice. Price per square inch settles the argument; the crust and the toppings complicate it.
Lena Hoffmann · 11 min read
Lumens are what a lamp emits; lux is what lands on a surface. You cannot convert one to the other without an area, or without a distance and a beam angle. Anything that offers you a bare conversion factor is selling you a fiction.
Lena Hoffmann · 11 min read
Sixty-three days is measured from ovulation, not from the mating. Counted from a mating date the same pregnancy spreads over two weeks, because sperm survive for days and a bitch is often mated more than once. What the calculator does with that, and where its window stops four days early.
Marco Bianchi · 11 min read
Resting energy scales with weight to the power 0.75, not linearly, which is why a chart that doubles the food for a cat twice the weight is 83 kcal a day over. Neutered against intact, indoor against outdoor, and the boundary the arithmetic must not cross.
Marco Bianchi · 11 min read
The depth follows the job: a bare display, a planted tank whose roots need room, an under-gravel plate. Then comes the number people are unprepared for — substrate is what puts a filled tank past what a piece of furniture can hold. Plus why fine sand compacts and coarse gravel traps waste.
Marco Bianchi · 12 min read
Two to four inches, and the right end of that window depends on the material: bark takes 3, screened compost takes 1 to 2, and gravel takes 1. Here is the depth trade-off, the settling nobody buys for, and the bag conversion where the count goes wrong.
Marco Bianchi · 13 min read
The same 2,000 sq ft lawn at the same 1 lb of nitrogen per 1,000 sq ft takes 40 lb of a 5 percent blend or 4.3 lb of urea. Target nitrogen, not product — and read the bag count as shopping, not spreading.
Marco Bianchi · 12 min read
The same tank needs a 100 W heater in a warm flat and 300 W in an unheated room. The variable is the gap between the water and the room, which most rules of thumb drop entirely. Plus why two smaller heaters beat one big one, and where this calculator's recommendation stops being trustworthy.
Marco Bianchi · 12 min read
Every stocking calculator implements a length-per-volume rule, and waste output scales with body mass, not length. A fish twice as long makes nearly five times the ammonia. Run through the tool's five methods, the same tank comes back holding anywhere from 6 to 12 fish.
Marco Bianchi · 13 min read
A normal medium load in a high-efficiency machine works out at 37 ml. Load size swings that threefold; water hardness only 1.28-fold — and the tool's hardness input carries no units at all. Here are the scales and the conversions it leaves out.
Marco Bianchi · 12 min read
Vout = Vin·R2/(R1+R2) is only true when nothing is connected to the output. Draw current and the voltage collapses, by an amount that follows one exact law. Here is that law, the ten-times rule checked against it, and why a lower-resistance divider costs you power forever.
Lena Hoffmann · 15 min read
Series adds, parallel adds reciprocals — that part is easy. The parts that catch people: which member dominates the answer, why two 5 % resistors in parallel are still 5 %, and why the smallest resistor in a parallel pair takes the largest share of the power.
Lena Hoffmann · 15 min read
Since June 2026 a French rooftop is paid 1.1 cents for an exported kilowatt-hour and avoids 25.61 cents by consuming the same one. That ratio of 23 to 1 makes the self-consumption rate — the variable almost every quote omits — worth more than the amount of sun falling on the roof.
Marco Bianchi · 11 min read
Before any grant, the price of electricity divided by the price of gas sets a seasonal efficiency the heat pump must beat just to break even on running cost. In late 2025 that number was 1.53 in Portugal and 2.79 in Germany — and on old radiators a German heat pump can cost more to run than the boiler it replaced.
Marco Bianchi · 11 min read
A compost heap wants 25 to 30 parts carbon to one part nitrogen by dry weight. Cardboard is 350:1, grass clippings 17:1. Here is the materials table, one mix worked through to 28:1, and why the volume rule of thumb is a different measurement entirely.
Marco Bianchi · 8 min read
A 750 ml bottle pours five glasses, and guests average about one drink per hour. Everything else is arithmetic — here it is for 50 to 250 guests over three, four and five hours, with an honest range rather than a single number.
Marco Bianchi · 7 min read
A conventional wet brine is 5 to 6 percent salt by weight of the water and you pull the meat out on a timer. An equilibrium brine is 1 to 2 percent by weight of the meat and water together, and it cannot over-salt. Those are different numbers doing different jobs.
Marco Bianchi · 9 min read
Multiply fresh yeast by 0.4 for active dry and by 0.33 for instant. Those two numbers are the whole chart, they are King Arthur's professional figures, and they are approximate — here is how far, and what else changes when you swap.
Marco Bianchi · 7 min read
Flour is 100 percent and everything else is measured against it. That one convention lets the same recipe scale from a single loaf to a bakery batch without a single new calculation — and yes, the percentages are supposed to add up to more than 100.
Marco Bianchi · 7 min read
10-20-10 does not mean 10 percent phosphorus. The second and third numbers are oxides, so a 50 lb bag holds 4.4 lb of actual phosphorus, not 10. Here is what each number really measures and how to turn it into a rate.
Marco Bianchi · 6 min read
For sound, f' = f(v + v_o)/(v − v_s) — and getting the signs backwards is the classic error. Here is the convention spelled out, a 440 Hz source computed at four speeds, and why light needs a different equation entirely.
Lena Hoffmann · 8 min read
Drop 25 °F and cut the time by about a fifth — then adjust for what the food is and how full the basket is, because those two change the answer more than the rule does.
Marco Bianchi · 7 min read
Temperature sets doneness, thickness sets time, and time at temperature is what makes food safe. Those are three different clocks, and most charts print only one of them.
Marco Bianchi · 6 min read
Re = ρvL/μ compares inertia with viscosity, and the units really do cancel. See the number worked out for honey, a household pipe, an artery, a swimmer and a wing — and why the 2 300 threshold belongs to pipe flow alone.
Lena Hoffmann · 9 min read