Anchor Bolt Pattern Calculator
Match a new pole to the bolts already in the ground.
Measure the existing bolts, type one number, and get the bolt circle and bolt spacing for a 4-bolt pattern. Free to use. No sign-up.
Quick answer for 4 evenly spaced anchor bolts
Spacing = Circle × 0.70711
Circle = Spacing × 1.41421
Example: 15 × 0.70711 = 10.607 in, about 10 5/8 in
Bolt pattern calculator
Model
Illustration, not to scale except for the bolt pattern. Red is what you typed. The other number is calculated. Drag sideways to turn the model.
Replacing a pole on existing anchor bolts?
- Match the bolt circle to the new base plate, and check the slot range on the new plate.
- Match the bolt size, and make sure the bolts stick up far enough above the concrete for the nuts, washers and base plate.
- Compare the new pole's EPA and wind rating to what will be mounted on it.
- Check the pole's orientation against the bolt pattern and the hand hole.
- Look at the condition of the existing bolts and threads.
How it works
The bolt circle is the distance across the pattern, from the center of one bolt to the center of the bolt diagonally opposite. The bolt spacing is the distance between the centers of two neighboring bolts, which is one side of the square.
With 4 evenly spaced bolts, the spacing is always the bolt circle times 0.70711, and the bolt circle is always the spacing times 1.41421.
Outside to outside adds one bolt width to each measurement. The calculator takes the bolt size off what you type, does the same math, and adds it back to the answer.
15 in bolt circle: 15 × 0.70711 = 10.607 in spacing, about 10 5/8 in.
10 in spacing: 10 × 1.41421 = 14.142 in bolt circle, about 14 1/8 in.
Common bolt circle sizes
Bolt spacing for 4 evenly spaced anchor bolts, measured center to center. Measured the spacing instead? Find it in the middle column and read the bolt circle on the left, or type it into the calculator above.
| Bolt circle (in) | Bolt spacing (in) | Nearest 1/16 in |
|---|---|---|
| 8 | 5.657 | 5 11/16 |
| 8 1/2 | 6.010 | 6 |
| 9 | 6.364 | 6 3/8 |
| 9 1/2 | 6.718 | 6 11/16 |
| 10 | 7.071 | 7 1/16 |
| 10 1/2 | 7.425 | 7 7/16 |
| 11 | 7.778 | 7 3/4 |
| 11 1/2 | 8.132 | 8 1/8 |
| 12 | 8.485 | 8 1/2 |
| 12 1/2 | 8.839 | 8 13/16 |
| 13 | 9.192 | 9 3/16 |
| 13 1/2 | 9.546 | 9 9/16 |
| 14 | 9.899 | 9 7/8 |
| Bolt circle (in) | Bolt spacing (in) | Nearest 1/16 in |
|---|---|---|
| 14 1/2 | 10.253 | 10 1/4 |
| 15 | 10.607 | 10 5/8 |
| 15 1/2 | 10.960 | 10 15/16 |
| 16 | 11.314 | 11 5/16 |
| 16 1/2 | 11.667 | 11 11/16 |
| 17 | 12.021 | 12 |
| 17 1/2 | 12.374 | 12 3/8 |
| 18 | 12.728 | 12 3/4 |
| 18 1/2 | 13.081 | 13 1/16 |
| 19 | 13.435 | 13 7/16 |
| 19 1/2 | 13.789 | 13 13/16 |
| 20 | 14.142 | 14 1/8 |
Frequently asked questions
What is a bolt circle?
The bolt circle is the diameter of the imaginary circle that passes through the center of every anchor bolt. On a 4-bolt pattern it is the distance from the center of one bolt to the center of the bolt diagonally across from it.
How do I measure the bolt circle on an existing foundation?
By default the calculator uses center to center: from the center of one bolt to the center of the bolt diagonally across from it. If you cannot find the center of a bolt, measure from the outside edge of one bolt to the outside edge of the bolt diagonally across, switch the calculator to outside to outside, and check the bolt size. It takes off one bolt width for you. You can also measure the spacing between two neighboring bolts and type that in instead. Measuring both and comparing them is a good way to catch a mistake.
Why does the bolt pattern matter when replacing a pole?
The base plate on the new pole has slots or holes for the anchor bolts. If the bolt circle of the existing foundation does not fall within the range of the new base plate, the pole will not sit on the existing bolts. Matching the pattern to the new base plate drawing is the first thing to check.
What other numbers should I get from the pole spec sheet?
Ask for the base plate drawing, which shows the bolt circle or slot range, the bolt size and the plate thickness. Also look for the pole height, the shaft size, the EPA rating, the wind speed rating and the drilling pattern for the arms or fixtures. Compare them to the existing pole and to what you plan to mount.
For reference only. Not for construction.
This calculator and the PDF it makes are for reference only. They are not construction documents. The results are only as accurate as the measurements entered, and the person who takes the measurements is responsible for them. Before ordering a pole, confirm the bolt circle, bolt size and projection against the pole manufacturer's base plate drawing, and against the engineer's foundation drawings if there are any. A+ Lighting Solutions is not responsible for errors that come from the measurements entered.
The PDF has a line for the name, signature and date of the person who took the measurements.
Need a pole quote?
Send us your sheet or give us a call. We will quote a pole that fits your bolt pattern.