The spacing that matters is centre to centre, not the gap
People lay stepping stones by choosing a gap that looks right and then setting the stones at that gap. The foot does not care about the gap. It lands at a distance from the previous footfall, and that distance is your pace. So the number to set out is the distance between stone centres, and the gap is whatever is left after the stone occupies part of it.
This inverts the usual habit in a useful way. Change to a bigger stone and the correct answer is a smaller gap at the same centres, not the same gap at wider centres. Wider centres would mean stretching, and stretching is what makes a path feel like an obstacle course rather than a route.
Getting your own pace instead of a table figure
Published average step lengths exist and they are not much use for one path in one garden. Pace varies with leg length, with speed, with what you are carrying, with the surface and with the slope. The way to get the right number takes about thirty seconds: walk the actual route at the speed you would normally use it, count your steps, and measure the distance you covered. Distance divided by step count is your pace length.
Walk it more than once and average. If two people use the path and their paces differ, use the shorter of the two, because a short-paced walker stretching to reach the next stone is worse off than a long-paced walker taking a slightly short step. If the path is going to be used at night or in the wet, take the pace at the speed you would walk it then, which is slower and shorter than your daytime pace.
Fitting the spacing to the run
A run rarely divides exactly by your pace. The calculator offers two answers. Fitting the spacing spreads the leftover across every gap so that both ends of the path land on a stone and the spacing is uniform, at the cost of every step being slightly off your natural pace. Holding the pace keeps every step exact and leaves an odd distance at one end, which you then have to deal with by shortening the path, moving the end stone, or letting the last two gaps take the difference.
Fitting is usually right when the adjustment is small, and the calculator will tell you when it is not small. Once the fitted spacing drifts more than about a fifth away from your pace, the run is between two stone counts and the honest fix is to change the run rather than the spacing. Paths that start and end at fixed points, such as a door and a gate, are the ones where this bites, and the usual solution is a wider landing stone at one end that absorbs the difference.
What goes under a stepping stone
A stepping stone that rocks is worse than no stone. Each one sits on a bed of compacted sand or screenings over ground that has been dug out to take the stone plus the bed, so that the stone finishes at or just below the surrounding grade. Setting them proud of the lawn makes them a mowing hazard and a trip hazard.
How deep the bed needs to be is a soil question rather than a stone question. Well drained sandy ground under foot traffic asks for very little. Clay that holds water, or ground that freezes, will move a shallow-bedded stone in the first winter and keep moving it every year after. Vehicle traffic is a different category entirely and is not what these are for. There is no depth this page can give you that is right for every site, and a bed that turns out to be too thin is a lift-and-redo rather than a top-up.
Before you dig, even for something this shallow, have the buried services located. The free national locate service covers every state, it costs nothing, and shallow irrigation lines, low-voltage lighting cable and gas services to outbuildings are exactly the things that run across the routes people put paths on. A struck line is a life-safety event, not an inconvenience.
Where this sits next to the harder surfaces
If the stones end up nearly touching, you are building a walkway and should treat it as one, with a continuous compacted base under the whole thing rather than a bed per stone. That is the territory of the paver patio calculator for quantities and the cut waste calculator if the walkway is patterned. For the fall across a path that runs alongside a building, or a path that has to shed water rather than pond on it, work the drop out on the hardscape slope calculator before you set the first stone.
Questions people ask
How far apart should stepping stones be?
At your own pace length, measured centre to centre, which is the whole point of measuring rather than copying a figure. Walk the route, count the steps, divide. That said, if you want a sanity check on the answer the calculator gives you: a spacing that comes out wildly different from the distance you naturally step is a sign that the input pace was taken at the wrong speed, on the wrong surface, or on the flat when the path is on a slope.
Should each stone take a left foot and a right foot alternately?
For a single-file garden path, yes, and that is what spacing at one pace length gives you: consecutive footfalls land on consecutive stones. Some designs instead place a stone for every second footfall, which doubles the spacing and is only comfortable for people with a long stride. A few wide paths pair stones side by side so either foot can land on each. Decide which you are building before you set out, because the three produce very different stone counts for the same run.
What about a curved path?
Measure the path length along the curve, not across it, and lay a rope or hose along the intended route to get that figure. On the curve itself, the inside foot travels a shorter distance than the outside foot, so stones on a tight bend are set slightly closer than the straight-run spacing. Set the straight sections from the calculator, then adjust by walking the curve and marking where your feet actually land.
How thick should the stones be?
Thick enough not to crack when someone lands on the unsupported middle of one, which depends on the stone and the span rather than on a single number. Thin flagstone bedded fully on compacted material can be surprisingly durable; the same stone bridging a void will break. What matters more than thickness is that the bedding is continuous under the whole stone. If a stone rocks, the bed is wrong and no thickness fixes it.
Can I put stepping stones through a driveway or where a car crosses?
Not as designed here. A stone bedded on a couple of inches of sand for foot traffic will be pushed into the ground the first time a wheel crosses it, and the edges will crack. Anything carrying vehicles needs a continuous compacted base built for that load, and how deep that base has to be depends on the soil and the vehicle. Treat a crossing point as a driveway problem rather than a path problem.