Rebar Chair Calculator

When pouring a concrete slab, the placement of your reinforcement is critical. If your rebar sits at the bottom of the pour, it offers very little structural benefit. To keep your steel suspended at the correct height, you need rebar chairs. Use the calculator below to estimate exactly how many chairs you need for your project.

Why Rebar Chairs are Essential for Concrete Longevity

Concrete is incredibly strong in compression but relatively weak in tension. That is where rebar comes in. However, for rebar to do its job, it must be embedded within the "tension zone" of the concrete—usually near the middle or bottom third of the slab, but never touching the ground.

Rebar chairs (or bolsters) serve three primary functions:

• Proper Positioning: They ensure the steel remains at the depth specified by structural engineers.
• Corrosion Prevention: By keeping the steel away from the soil, they prevent moisture from wicking up and rusting the metal, which leads to concrete "spalling."
• Stability: They hold the grid in place so it doesn't shift when workers walk on the mesh or when the heavy wet concrete is poured.

How to Calculate Your Requirements

The general rule of thumb for rebar chair placement is to space them every 2 to 3 feet along the rebar. If you are using lighter mesh (WWM), you may need them closer together to prevent sagging. For heavy-duty #5 or #6 rebar, you can often push the spacing to the 3-foot limit.

Types of Rebar Supports

Depending on your project, you might choose different types of supports:

• Plastic Individual Chairs: Most common for residential slabs and driveways. They are lightweight and won't rust.
• Concrete Bricks (Dobies): Often used in heavy commercial foundations or when the slab is poured directly on vapor barriers or soft soil.
• Metal Bolsters: Long continuous runs of wire support used for bridge decks or large commercial floors.

Pro-Tips for Installation

When setting up your grid, always start your chairs approximately 6 inches from the edge of the slab. Ensure the "feet" of the chairs are resting on a stable surface. If you are pouring on top of a moisture barrier, use chairs with a wide base (sand plates) to prevent the chair from puncturing the plastic film.

Remember: It is always better to have a few extra chairs than to have a sagging rebar grid during the middle of a high-stakes pour.