How it measures

Every number in Paddaro comes from a sensor, a public data source, or arithmetic on those. This page says which, and how sure we are. Three words carry that throughout:

• Measured comes straight from a sensor or a public gauge.
• Derived is arithmetic on measured values.
• Inferred rests on a model or an assumption that hasn't been fully validated yet.

Stroke counts and sides are inferred. They are marked beta in the app until they've been checked across many paddlers, wrists and boards.

What gets recorded

During a session the Apple Watch records:

• wrist motion (attitude, rotation and acceleration) 200 times a second, or 100 on watches without the faster stream
• GPS position and speed once a second
• heart rate every few seconds, through an Apple Health workout
• the moment you close a split with the Action button or on screen

The raw recording is kept, so when the methods below improve, past sessions can be worked out again without paddling them again.

Strokes

A paddle stroke makes your wrist repeat the same movement once per stroke. Paddaro filters the motion to the range paddling happens in (about 20 to 100 strokes a minute), finds the direction your wrist is moving most, whichever way the watch is worn, and counts a stroke at each crest of that motion. A crest has to be big compared with the last few seconds, and can't come too soon after the previous stroke.

The same counter runs live on the Watch and again on the iPhone after the session. On the iPhone, four filters then remove what isn't paddling: arm swing while walking, isolated movements, time spent kneeling or fiddling with gear while the board sits still, and carrying the board to and from the water.

Stroke rate is strokes per minute of paddling, leaving out pauses of 4 seconds or more. The Watch also checks the rate a second way, from how the motion repeats, and dims the live number when the two disagree. Distance per stroke is distance over strokes.

Which side

The hand nearer the blade points down the shaft. The top hand on the grip sits level or higher. So the angle of your watch forearm just before each stroke says whether the watch hand was the lower hand, which means you were paddling on the watch's side. Paddaro keeps a running average of the two positions as you paddle, and a side change counts once two strokes in a row land on the other side.

Turns

A turn is a stretch where your heading changes by 90 degrees or more while the board is moving at least 1 metre a second. Paddaro reports its direction, angle, entry and exit speed, the strokes on each side during it, and a radius from the path length over the angle. GPS at one fix a second makes tight turns read wider than they are: a step-back pivot seen on video came out at about 5 metres.

What a side change costs

Every side change outside a turn is lined up on one clock, and board speed is averaged second by second from 5 seconds before to 6 seconds after. The dip below the speed you had beforehand is the distance and time a change costs you. It needs at least 10 changes in a session.

Current

From your paddle. On an out-and-back, the route is cut into four bands by distance from the start. In each, half the difference between your speed coming back and going out is the current. This only reads as current if you worked equally hard both ways, which is why your heart rate is shown beside it. Wind ends up in the same number.

Ahead of time. Where a river reach is fed by a dam, Paddaro estimates current from the published release and the size of the channel. Future hours use the typical release at that hour over the past week. The channel size is estimated from a map and could be off by a factor of two, so treat the speed as a rough band: easy, moderate, hard or very hard.

Water grade and wind

Hourly wind comes from four forecast models. Paddaro uses the middle of them for the steady wind and their spread for gusts. The grade follows the Beaufort scale's edges on the steady wind: calm under 8 mph, light chop under 13, choppy under 19, rough above. It moves one step rougher when the strongest model gust falls in a higher band.

Beaufort describes open sea. On a lake the waves are smaller because the wind has less open water to build them, and trees or bluffs can block much of the wind. Each session page asks how the water actually was, and those answers are compared with the forecast at your spot.

Water temperature and quality

Water temperature comes from the nearest US Geological Survey gauge where one exists, or from a marine model on the coast. In California, Paddaro shows the most recent state bacteria samples within about 3 km and algal bloom reports within about 5 km, with their dates. Samples are often weeks old.

Heart rate efforts

Heart rate is smoothed onto a five-second grid. An effort runs from a low to the next high, where a high is confirmed once heart rate falls 12 beats a minute below it. Efforts shorter than 45 seconds or 15 strokes are left out. Recovery is how far heart rate falls in the minute after the peak. Wrist heart rate has not been compared with a chest strap.

What has been checked

So far, on one 51-minute paddle at Lake Natoma in October 2026, filmed from the board:

• Over six stretches of video, every side change appeared in Paddaro's output within about a second, on the correct side.
• Counted frame by frame, six pulls in ten seconds against six detected strokes.
• Rests and kneeling spells lined up with the footage to within about 20 seconds.

Not checked yet

• Stroke counts against a full hand count over a whole session
• Other paddlers, the right wrist, sprint pace, and rough water
• Wrist heart rate against a chest strap
• The current estimate from dam releases against a measured current

Known weak points: the first stroke after a rest is often missed, and movement while kneeling can still count as strokes. You can check your own count in a minute: close a split, take 30 strokes on one side, and close another.