How to measure slope for a backyard rink

You need four numbers, one per corner of the rink: how far the ground sits below one level line. They take about 15 minutes to get, and they decide how tall the boards must be at the low end. The calculator does the rest.

What you need

Four stakes, or four sticks you can push into the ground.
Mason string or another cord that does not stretch, long enough to reach between two stakes.
A line level (a few dollars at a hardware store) or a carpenter's level.
A tape measure, and something to write on.

Steps

Mark the corners. Drive a stake at each corner of the rink, where the inside of the boards will sit. Measure both diagonals. If they match, the stakes form a true rectangle.
Find the high corner. Look over the yard to see which corner sits highest. Start there.
Set the string height. Tie the string to the high stake about 12 in above the ground, high enough to clear the grass and any bump between the stakes. Every reading is taken from this level.
Level the string. Pull it tight to the next stake, hang the line level in the middle and raise or lower the free end until the bubble centers. A sagging string reads wrong, and the error grows with distance.
Measure down. At each stake, measure from the string to the ground and write it down. At the high stake that is about 12 in, since you set it there.
Reach the other corners. Move along the rink, tying the string off at a stake at the same height you just leveled to. If you re-tie the string at a new height, measure the high corner again so all four numbers come from one level line.
Mind long runs. A string over about 40 ft sags and is hard to level by eye. On a longer rink, level to a mid-point stake first and continue from a mark on it.

Enter the readings

Type each reading into the matching corner field. A bigger number means lower ground. Nothing needs subtracting: the calculator puts the water level at the depth you choose above the smallest reading and works out every other corner from there. Keep the same orientation as the length and width fields, where front to back is the length.

Example: a 40 by 20 ft rink with readings of 14 in at the front left, 15 in at the front right, 17 in at the back left and 18 in at the back right. The front left is the high corner, and the back right is 4 in lower. With 3 in of water at the high corner and 2 in of board above the water line, the deep end has 7 in of water and needs a 9 in board. The rink holds 2,494 gallons. Open this example in the calculator.

Another way: a straight board

Without a line level, lay a straight 8 ft board on edge between two points, set a carpenter's level on top and lift the low end until the bubble centers. Measure the gap under the lifted end. Repeat along the rink and add the gaps up. It takes more steps than the string and adds up small errors, so use it for short rinks or as a check on the string readings.

A rotating laser level also works. A phone level app is the least accurate option, so check it against a carpenter's level before you trust it.

Things that throw readings off

Bumps and dips between corners. The calculator treats the ground as a straight slope from corner to corner. If a hump or hollow sits in the middle, take an extra reading there and plan the boards around the lowest ground you find.
Long grass and leaves. Measure to the soil or to the mowed grass, and pick the same one at every stake.
Loose stakes. A leaning stake moves the string height. Push them in firmly and recheck the high corner at the end.
Frost and snow. Measure in fall, before the ground freezes. Snow hides the real surface.
A tilt across the width. A yard that drops along the length and also tilts sideways gives four different readings. That is why the calculator takes four corners, and each side gets its own board height.

Next: what the numbers mean for building on a slope, and the liner size that goes with taller boards.