Pwyso: how to calculate cattle weight without a scale

1. 01

   The problem

   Many small livestock keepers cannot put every animal on a scale when they need a planning estimate. A tape and a formula work, but a tape is hard, and not always safe, to hold around an animal that will not stand still. A single photo hides depth, so a one-photo estimate tends to show an exact-looking number it cannot back up.

2. 02

   The plan I wrote

   Pwyso uses two Android phones, a lead phone and a companion phone, on the same local network. A short pairing code links them. The keeper types in the real distance between the two cameras, and both phones photograph the animal’s side at one agreed moment.

   The lead phone then compares the two pictures, works out the size of the visible body, applies the body shape of the chosen species (cattle, sheep or goat) and gives a weight range. The center number comes second to the range, and the result lists what widened it. A “Truth lab” lets the keeper correct for one particular pair of phones by photographing an object of known size, such as a taped board or a gate opening.

   The photos go straight from one phone to the other and are kept only as temporary files. Accepted measurements stay on the lead phone, with no account.

3. 03

   The build

   AI coding agents built it from my written specification. My work was the specification and the rules for when the app must refuse: there is no success screen unless every check passes.

4. 04

   When it says no

   The app refuses to give a range, and says why, when:

   • the two shutters fired more than 80 thousandths of a second apart;
   • too few shared features were found in the two pictures;
   • the matches between the two pictures disagree too much for a stable depth;
   • the animal is too small, or fills too much, of the frame;
   • no clear side-on pose was found;
   • too much of the picture is lost to shadow or glare;
   • the measured size does not fit the chosen species.

   Then it tells the keeper what needs another capture.

5. 05

   What I have checked, and what I have not

   Automated tests feed the engine a computer-made pair of pictures with a known shape. It finds enough shared features, returns an ordered range, and refuses a pair whose shutters were 125 thousandths of a second apart.

   What I have not done: when I built it, no animal and no reference scale were available, so I make no claim about how close the range is on a real animal. No object of known size was photographed with both cameras either, so there is no real-world calibration result.

6. 06

   Where it stands

   “Pwyso: Livestock Weight” is live on Google Play. It gives model-based estimates, not scale readings. For medicine doses, a sale or transport, use a real scale or a qualified professional.

   If you keep cattle, sheep or goats and can weigh an animal on a real scale, I would like to hear how far the range was from the scale. Email me or message me on LinkedIn.

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