HoroCalc

Five precision calculators for mechanical watch and clock work — all offline, no accounts required.

MAINSPRING SIZER
Enter the barrel bore, arbor diameter, and barrel height to calculate the correct spring length, width, and thickness using the standard ⅓ barrel-fill rule. Enter the spring thickness if known for an exact
result.

BEAT RATE
Select from the six most common frequencies or enter any BPH value. See the balance frequency in Hz, beat interval in milliseconds, and beat period. Enter a measured daily rate error to see the deviation in
parts per million against the COSC chronometer standard.

PENDULUM
Convert between pendulum length and beat rate in both directions. Enter a measured daily rate error and the app calculates exactly how many millimetres to raise or lower the bob to correct it.

GEAR TRAIN
Enter tooth counts for the centre, third, fourth, and escape wheels and their pinions. The interactive diagram shows the full gear chain — tap any value to edit it. Instantly calculates beat rate, frequency,
beat interval, and fourth wheel RPM, with a green indicator when the fourth wheel hits 1 RPM for a seconds-hand watch.

POWER RESERVE
Enter barrel bore, arbor diameter, spring thickness, barrel rate, and efficiency to estimate maximum turns, useful turns, and total power reserve in hours.

Every calculator opens with pre-filled example values so results appear immediately. Works entirely offline.