How it is worked out
Arithmetic, notation conversion and age-based lookup are three different kinds of answer.
Height growth calculator
Both numbers go to centimetres first (1 inch = 2.54 cm), then subtract. Days are counted on the calendar rather than estimated as “about six months” — 1 March to 1 September is 184 days, not 180.
This is a difference between two numbers, not a growth-velocity percentile. It cannot tell you whether the growth was enough — that needs a velocity reference by age, and no such table is published here yet. If one measurement was lying and the other standing, part of the difference is the method rather than the child, and nothing here subtracts it for you.
Growth chart maker
One record per line: date first, value second, separated by a comma or a space; the `cm` is optional. Lines that cannot be read are listed back with their line numbers — never skipped in silence, because pasting ten lines and getting nine points with no explanation is worse than getting no chart at all. Two different values on one date are both kept and flagged; which one is right is something only you know.
Only your own records are drawn. Nothing is interpolated between points, nothing extrapolated past the last one, and no population bands are laid underneath — those would need a reference table, and drawing them here would turn “how this child has grown” into “how this child measures up”, which is a different tool.
Weight change calculator
Both numbers go to kilograms first (1 lb = 0.45359237 kg), then subtract. Days are counted on the calendar.
Gaining is not progress and losing is not failure. What you get is a signed number and a count of days, with no adjective attached and no plan to gain or lose. A child’s weight swings by a few hundred grams within a single day, so if the two weigh-ins differed in time of day, clothing or whether they had just eaten, part of the difference has nothing to do with growth.
Visual acuity notation converter
All three notations describe the same thing: the smallest angle the eye can resolve. Decimal is its reciprocal. Snellen 20/40 means “at 20 feet, reads the line a normal eye reads at 40”, so the decimal is 20÷40 = 0.5. logMAR is the base-10 logarithm of that angle. Nothing is looked up, which is why this page works today.
Rewriting the notation does not make two tests comparable. Charts differ in spacing, optotype (tumbling E, Landolt C, Snellen letters), lighting and test distance, and the same child can land a line apart on two of them. This page does not convert acuity to a spectacle prescription, does not decide whether a child is short-sighted, and does not grade anyone for referral — each of those rests on evidence of its own. The Chinese five-point scale (4.0–5.3) is deliberately absent: its baseline differs from logMAR, and folding it in would mislead.
Sleep duration calculator
Whether waking was the next day or the same day is something you tick, because it cannot be inferred: 21:30 to 07:00 is either nine and a half hours overnight or a typo, and 13:00 to 15:30 is a nap. Night-time and nap are given separately, and shown separately.
This is not sleep tracking. Lights-out is the time you wrote down, not a moment a device detected, so time in bed usually exceeds time asleep. What you get is how long the interval you recorded was — not how long the child slept, and certainly not whether it was enough. That last one needs a reference by age, and none is published here yet.
Nutrition label intake calculator
One multiplication and one division, using the label’s own item and unit. Grams and millilitres are never converted for you — a millilitre of oil and a millilitre of water are not the same thing.
“Total sugars” is not silently treated as free sugars, sodium is not converted to salt, and nothing is compared against a recommended intake — that last one needs a reference table by age, and none is published here yet.
The ones that read a table
Age, region, unit and source all have to line up. Where there is no table, nothing is extrapolated and no disclaimer stands in for a result — a light tool still has to show the conditions it holds under.