The blog

Long-form on trend maths, dose tracking and the strange business of measuring a body with a bathroom scale. New post every other day.

• Does oral semaglutide have the same half-life as the injection?About a week either way. The interval is what differs: a daily tablet ripples by about 10%, while a weekly injection swings by a factor of two.
• Shotsy vs Peptrend: GLP-1 dose logging and weight trend mathsShotsy is one of the two largest GLP-1 trackers, and its shot log and site rotation are the better half of it. It names no smoothing algorithm. Checked 18 Sep 2026.
• How fast is too fast to lose weight? The 1.5 lb/week capPeptrend refuses any plan faster than 1.5 lb a week and greys out Save. Where that cap sits against published rates, and how the rate, ETA and deficit are computed.
• Is it a real plateau? Why two weigh-ins a fortnight apart can't sayA stall is only a stall once it clears your own noise. On one user's log the two-morning read and the fitted slope disagreed on direction in 15 of 92 windows.
• How many units is 1 mg? Why a calculator should stop at mg/mLA unit is a mark on a syringe barrel, so 1 mg has no unit count until you name the barrel. Peptrend divides strength by water, prints mg/mL and stops there.
• How to import your weight history from Apple HealthOne tap reads every body-mass sample Apple Health holds, back to the first. Peptrend writes back only the weigh-ins you type in.
• Why the scale hasn't moved in 2 weeks: the signal-to-noise mathsTwo weeks is rarely long enough to tell a real change from ordinary scale noise. Work out your own noise-to-signal ratio, then how many days it takes to clear it.
• When does a GLP-1 peak after injection? Tmax and the absorption curveHours to days after the shot — 1 to 3 days for semaglutide, 8 to 72 hours for tirzepatide by the labels. The peak lands at D × F × 2^(−Tmax ÷ half-life).
• What one bad weigh-in does to your trend line (rolling median)One 10 lb error moves a 7-value rolling median 0.4 lb and a 7-value moving average 1.43 lb. Double the error and the mean's miss doubles; the median's does not.
• Is there a TrendWeight app for iPhone? TrendWeight vs PeptrendTrendWeight is web-only and free forever, takes typed weigh-ins or a Withings sync, and publishes every formula it uses. Peptrend is an iOS app with fifteen methods.
• Injection site rotation: how to keep track without a body mapRotation works when the order is decided in advance. Peptrend saves a site with every dose and suggests the next by round-robin through your list — no body map.
• Why your trend line follows every weigh-in (Hull moving average)Alan Hull's filter subtracts its own lag until the line leads the scale rather than trailing it. On 84 real weigh-ins it ignores 14.3% of daily movement.
• Does Happy Scale log GLP-1 doses? Happy Scale vs PeptrendHappy Scale does not advertise injection or dose logging and does not publish the formula behind its recommended default. Peptrend publishes fifteen and logs doses.
• What your trend does on days you skip a weigh-in (interpolation)On a day you did not weigh, the app draws a straight line between your two nearest readings and hands it to the filter — 142 of 226 days on one user's log.
• Why one trend method sits 4 lb above the scale (Kaufman KAMA)KAMA squares its smoothing constant, capping the alpha near 0.048. On 84 real weigh-ins it ignores 81.3% of daily movement and ends 4.0 lb above the last weigh-in.
• How to chart a titration, and why the scale lags a dose stepPeptrend colours each segment of the weight line by the dose behind it, so a 1.2 to 3.5 mg ladder reads as a four-colour ramp. Why a step takes weeks to land.
• Why your weekly rate changes every day (the Kalman filter)The Kalman method carries your rate of change as part of its state and updates it with every weigh-in, so the weekly figure moves daily. Measured on 84 weigh-ins.
• How much bacteriostatic water? Reconstitution is one divisionThere is no single right amount — the water you add sets the strength. A 6 mg vial with 3.0 mL is 2.00 mg/mL; the same vial with 0.7 mL is 8.57 mg/mL.
• How long until your weight trend catches up? 28 days, or 5An EWMA at α = 0.1 needs 28 days to absorb 95% of a real change. DEMA gets there in 8 days and TEMA in 5 by subtracting an estimated lag, and both overshoot.
• Why your trend line lags behind the scale (SMA vs EWMA)A trend line lags because it averages weigh-ins you already took. At the shipped defaults a 7-day SMA sits 3 days back and an EWMA at alpha 0.10 sits 9 days back.
• How long until a GLP-1 reaches steady state? Four to five half-livesFour to five half-lives — 28–35 days for semaglutide, 24–30 for retatrutide, 20–25 for tirzepatide. Why weekly doses stack, and what 'in your system' counts.
• Why your weight trend reads higher than the scale (Hacker's Diet)A trend line that skips the days you missed takes fewer steps down, so it sits above your weight: 186.0 lb against a last reading of 183.0 on one user's log.
• Can you use trading indicators on your weight? DEMA, HMA, KAMAThe price-only ones, yes. DEMA, TEMA, HMA and KAMA read one number per period, so nothing in them assumes a market. On one real log they finish 4.0 lb apart.
• Why your weight trend barely moves when the scale jumps (EWMA)Your trend line travels a tenth of the way toward each new weigh-in, so a 5.4 lb morning shifts it 0.54 lb. What alpha does, and why a shock halves in 6.6 days.
• What should you log with every GLP-1 dose? Nine fieldsNine typed fields: time, medication, milligrams, route, site, vial, taken, pain and colour. What each one lets you answer later, and why there is no notes box.
• Why did I gain weight overnight? Water, glycogen and gut contentsAn overnight gain is water, glycogen, gut contents and sodium. In one 84-weigh-in log the daily swing ran 11.9 times the real rate of change underneath it.