Compare recorded weights from two seven-day periods using the weekdays present in both. See how that matched comparison differs from averaging every available reading.

Clear the log or Reset before changing its unit.

Each period covers its start date plus 6 calendar days. Periods must not overlap. Differences are B minus A.

One reading per date, within either period; at least one in each. Up to 14 rows, 6 decimal places, and 1,000,000 in the selected unit.

After Calculate, this log is saved in this browser. Other scripts on this site may access saved values. Reset clears this calculator’s saved data.

How to use the calculator

  1. Choose the unit used by every reading: kg or lb.
  2. Enter two nonoverlapping period starts. Each period includes its start date and the next six calendar days.
  3. Paste one YYYY-MM-DD, weight row per recorded date, with at least one reading in each period.
  4. Select Calculate. Open Calculation details for the seven weekday rows and missing-date coverage.

The starts can fall on different weekdays. Monday pairs with Monday, rather than with the same position in each period. Differences always mean period B minus period A, even if B is earlier.

Calculation

Each all-record mean is the sum of that period’s readings divided by its recorded-date count. For the matched comparison, let S contain only weekdays recorded in both periods:

Matched difference = Σ(Bd − Ad) / |S| = matched mean B − matched mean A.

Ad and Bd are the weights recorded on weekday d in periods A and B. |S| is the number of shared weekdays; means and differences use the selected kg or lb unit.

Missing dates are omitted, never filled with zero or estimated. With no shared weekdays, the matched result is unavailable. Positive means B is higher; negative means B is lower. The arithmetic uses entered decimals without intermediate rounding; ≈ identifies a display rounded to nine decimal places.

Example with different missing weekdays

For periods starting October 5 and October 12, 2026, suppose A has Monday 80.8, Tuesday 80.4, Thursday 80.0, and Friday 79.8 kg. B has Monday 80.3, Wednesday 80.0, Thursday 79.9, and Friday 79.7 kg.

All-record means are 80.25 and 79.975 kg: a −0.275 kg difference. The three shared weekdays are Monday, Thursday, and Friday. Their differences are −0.5, −0.1, and −0.1 kg, giving a matched difference of −7/30 kg, approximately −0.233333333 kg.

What the comparison can tell you

This describes the supplied samples. It does not establish fat loss, a trend, a healthy rate, or a treatment decision. Matching weekdays cannot remove changes in weighing conditions or explain missing readings.

NHS England’s recording guidance recommends consistent timing, scales, placement, and clothing. A 2020 weight-maintenance study observed weekday patterns in its participant group; it did not validate this two-period calculator or an individual correction. The tool does not recommend taking additional readings.

Frequently asked questions

Does matching make the result more reliable?

It makes the weekday sets the same, while potentially using fewer observations. Coverage counts are not confidence scores; even one shared weekday is shown without a trend claim.

Can I mix pounds and kilograms?

No. Every row must use the selected unit. Clear the log or Reset before changing it. Pounds and kilograms are distinct units; NIST defines one avoirdupois pound as exactly 0.45359237 kg. This tool does not convert the log.

What inputs are supported?

Real dates from 1900 through 2100, up to 14 unique dated readings, and positive decimal weights up to 1,000,000 in the selected unit with six decimal places. These are technical limits, not health recommendations. Overlapping periods, repeated dates, and readings outside both periods are rejected.