Concrete Forming Basics

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Welcome to Concrete Forming basics

To set up concrete forms the right way, you only need a handful of essential tools. These are the basics:

Metal stakes
2x4s or other straight boards (depending on what you’re forming)
Sledgehammer (for driving stakes)
Duplex nails (8ds and 16ds)
Tape measure
Claw hammer
String line (for straight, level edges)
Level/Torpedo Level (for checking grade and keeping forms true)
Wood saw (if you’re not using metal forms)

These tools are the foundation of clean, accurate forming — whether you’re doing a small walkway or a full driveway.

How to Set Your Concrete Forms

Setting forms is all about getting your boards straight, solid, and at the right height. Start by placing your first form board where you want the slab to begin. This board becomes your reference. Drive a couple of stakes behind it and lightly fasten the board so you can still adjust it.

Run a string line from that board to the opposite end of your slab. Set your second board to match the string, keeping it straight and at the correct height. Use your torpedo level to make sure the board isn’t twisted or leaning. Once both boards are lined up, stake them tight and nail them off.

Repeat this process for the remaining sides, checking your grade, pitch, and diagonals as you go. When all four sides are straight, level, and square, your forms are ready for reinforcement and pour day.

Getting Your Forms to Grade

Getting your forms to the right grade is one of the most important steps in concrete work. Start by setting your first board at the exact height you want your finished slab. Use a string line and torpedo level to carry that height around the rest of your forms.

For drainage, always give your slab a slight pitch so water runs away from the house or structure. A good rule of thumb is ⅛–¼ inch per foot. Set your string line to that drop, then adjust your boards until they match the pitch.

Once everything is lined up, stake your forms solid, re‑check your heights, and make sure your diagonals match so the slab is square and drains correctly.

Basic Math for Squaring Your Forms

Using 3‑4‑5 and 6‑8‑10 to Square Your Forms

When you’re forming concrete, getting your slab perfectly square is critical. The easiest way to do that is by using the Pythagorean theorem, which says:

A2+B2=C2

This is where the 3‑4‑5 rule comes from — and why it works every single time.

Why 3‑4‑5 Makes a Perfect Square

If one side of your corner is 3 feet, the other is 4 feet, the diagonal between them will be exactly 5 feet when the corner is a perfect 90° angle.

3² = 9
4² = 16
9 + 16 = 25
√25 = 5

So if your diagonal measures 5 feet, your corner is square.

The 6‑8‑10 Method (Same Rule, Bigger Scale)

The 6‑8‑10 method is just a larger version of 3‑4‑5. It’s more accurate on bigger slabs like driveways and patios.

6² = 36
8² = 64
36 + 64 = 100
√100 = 10

So if your diagonal is 10 feet, your corner is perfectly square.

When to Use Each

3‑4‑5 → small pads, steps, walkways
6‑8‑10 → driveways, patios, garages, larger slabs
Bigger numbers = more accuracy over long distances

If Math Isn’t Your Thing

You can also my Hypotenuse Calculator. Just plug in your rise and run, and it will calculate the hypotenuse for you so you can square your forms fast and accurately.

How to Check Diagonals Using the Hypotenuse

When your forms are set, the fastest way to make sure your slab is perfectly square is to measure corner‑to‑corner diagonals. If both diagonals match, your forms are square.

Below is a simple diagram showing how it works.

Diagonal Checking Diagram

   A (Corner)                         B (Corner)
      ●------------------------------●
      |                              |
      |                              |
      |                              |
      |                              |
      ●------------------------------●
   D (Corner)                         C (Corner)

Measure diagonal AC
Measure diagonal BD
If AC = BD → Your slab is square

How the Hypotenuse Comes In

Each diagonal is the hypotenuse of a right triangle formed by your form boards.

You’re basically checking:

A2+B2=C2

Where:

A = length of one side
B = length of the other side
C = diagonal (hypotenuse)

If the diagonals match, the corners are 90°.

Example Using 3‑4‑5 or 6‑8‑10

Let’s say your slab is 12 ft by 16 ft.

You can scale the 3‑4‑5 rule:

Multiply each number by 4
3×4 = 12
4×4 = 16
5×4 = 20

So your diagonal should be 20 ft.

If both diagonals measure 20 ft, your forms are square.

Diagram With Hypotenuse Shown

      |\
      | \
   B  |  \  C (Diagonal / Hypotenuse)
      |   \
      |____\
         A

A = one form board
B = the other form board
C = diagonal you measure

How to Do It in the Field

Set your forms roughly square
Measure corner to corner (diagonal #1)
Measure the other diagonal (diagonal #2)
Adjust your forms until both diagonals match
Re‑check after staking and nailing

This is the most accurate way to square a slab — period.