Measuring Your Doors: The Grain and Bolt Survey Guide
Getting the measurements right is the most important part of buying a new door. While it might seem a bit technical, taking a few extra minutes now ensures that when your Grain and Bolt door arrives, it fits like a glove and stays smooth for years.
In this guide, we’ll show you how to “audit” your door frame just like a professional joiner would.
1. Is your frame square? (The Diagonal Test)
Most people just measure the width, but frames can sometimes “lean” or “rack” over time.
- The Quick Check: Measure diagonally from the top-left corner to the bottom-right, then from the top-right to the bottom-left.
- What to look for: If those two numbers are more than 5mm apart, your frame isn’t perfectly square. Don’t worry—it just means you’ll need to allow for a little extra trimming during installation to get those perfect, even gaps.
2. Is the frame “plumb” and “true”?
- The “Plumb” Check: Put a spirit level against the side of the frame. If it’s leaning in or out, your door might want to “creep” open or shut on its own.
- The “Twist” Check: Check that the two sides of the frame are parallel. If one side is twisted compared to the other (we call this being “out of wind”), the door might hit the frame at the top before it’s fully closed at the bottom.
3. Taking the Measurements
Forget your old door for a second—we want to measure the space the new door will live in. Measure the inside of the frame (the opening) in these spots:
- Width: Measure at the Top, Middle, and Bottom.
- Height: Measure on the Left and Right sides.
- The Rule: Always use the smallest measurement when choosing your door size. We will help you factor in the 3mm gap needed at the top and sides for a smooth swing.
4. Thinking About the Floor
Are you planning on new flooring?
- If you’re measuring a bare sub-floor but plan on adding thick underlay and a plush carpet, remember that the “height” of your opening will decrease.
- We usually recommend a 10mm gap (the “undercut”) at the bottom of the door to allow for air circulation and to clear your carpets.
Your Handy Survey Checklist
Keep these numbers nearby when you’re browsing our shop.
| Where to measure | Measurement (mm) | Expert Tip |
| Width (Top) | Is the frame wider at the top than the bottom? | |
| Width (Middle) | Check here to see if the frame “bows” in. | |
| Width (Bottom) | Measure near the floor. | |
| Height (Left) | Measure from the floor to the top of the opening. | |
| Height (Right) | Floors aren’t always level—check both sides! | |
| Diagonal A | Top Left to Bottom Right. | |
| Diagonal B | Top Right to Bottom Left. |
Three Things to Remember:
- The Smallest Number Wins: Always order based on the tightest part of the frame. It’s much easier to trim a door down than it is to make one grow!
- Floor Clearance: If your floor is uneven, measure to the highest point of the floor so the door doesn’t catch.
- Walls & Skirting: Take a quick look at the wall thickness too. This helps you ensure the door “lining” (the frame) matches the depth of your walls perfectly.