Guide
Control joints in brief
Concrete shrinks as it dries. It also moves with temperature. A crack will show up somewhere. A control joint is a planned groove so that “somewhere” is a straight line you can live with, not a random map across the patio.
Two different joints
A control joint (sometimes called a contraction joint) weakens the slab on purpose so it cracks at the groove. It is sawed or tooled while the concrete can still take the cut. An isolation joint is a separator against a wall, a stair, or a column that moves on a different schedule. They are not interchangeable.
Reinforcement that crosses a control joint the wrong way can fight the joint. That is a layout question. The reinforcement note names the steel. This page will not space it.
Why there is no spacing calculator here
Rules of thumb exist in the field: so many feet between joints, a panel that is not too long and skinny, a joint that lines up with a change in width. Those rules depend on thickness, mix, weather, and what the city or the plan already wrote. A number typed into Pour Plan would look like a design. It would not be one.
If a sidewalk or an approach has a published joint pattern, use that pattern. If a patio is a private pad with no drawing, ask the person who is finishing the slab how they will cut it. Yardage does not change when you add a joint. The slab calculator still reports the same volume.
What joints do not fix
A joint will not hide a slab that is thin in the middle, poured on frozen ground, or sloped into the house. Those problems sit on the checklist and the subgrade note. Joints manage cracking. They do not replace thickness or drainage.
This is a definition, not a joint plan. Pour Plan does not mark the slab and does not sell a saw.