A door frame that isn't straight causes problems that only get worse over time — doors that won't close properly, gaps that let in draughts, and trim that never sits flush. Getting it right from the start comes down to two things: the frame must be plumb (perfectly vertical on both sides) and the head must be level (perfectly horizontal across the top).
Tools and materials for installing a door frame
The door frame components (two side jambs and a head jamb), timber shims, screws or nails, a drill, a hammer, a tape measure, and your phone with Spirit Level Online open in the browser.
Understanding plumb vs level
For a door frame, you need to check two things separately. Plumb means each vertical side jamb is perfectly upright — not leaning forward, backward, left, or right. Level means the horizontal head jamb across the top is flat, not sloping to one side. Both must be correct for the door to hang and operate properly.
Step-by-step: installing the frame straight
Set the head jamb first. Position the head jamb (the horizontal top piece) across the top of the opening. Hold your phone flat against the underside of the head jamb and open Spirit Level Online in Surface Mode. The bubble should sit at centre. Shim beneath one end until the reading reaches 0.0°, then fix it in place.
Check the first side jamb for plumb. Stand the first side jamb in position. Switch to Wall Mode on Spirit Level Online and hold your phone flat against the face of the jamb. The bubble should centre at 0.0°. If the jamb leans, insert shims behind it at the top or bottom to bring it plumb.
Check the side jamb in both directions. A jamb can be plumb front-to-back but still twist left-to-right. Hold your phone against the side edge of the jamb as well. Both faces need to read 0.0° before you fix it.
Fix the first jamb and repeat for the second. Once plumb in both directions, screw or nail through the jamb into the rough opening, securing the shims as you go. Repeat the same plumb check on the second side jamb.
Check the opening is square. Measure diagonally from the top-left to the bottom-right corner of the frame opening, then from the top-right to the bottom-left. If both measurements match, the frame is square. If they don't, adjust the shims on one jamb until they do.
Final check before hanging the door. Run through all three checks one more time — head jamb level, both side jambs plumb in both faces. Only then hang the door.
Tip: Camera bump calibration. Most modern phones have a protruding camera lens that stops them lying perfectly flat against a surface. In Wall Mode, press the Zero button while holding your phone against the jamb before taking your reading. This calibrates out the camera bump and gives you an accurate plumb measurement.
What if the rough opening itself isn't square?
Rough openings in older buildings are rarely perfectly square. This is exactly why shims exist — they let you build a square, plumb frame inside an imperfect opening. Pack shims behind each jamb at the top, middle, and bottom until the spirit level reads true, regardless of what the surrounding wall is doing. The door only needs the frame to be right; it doesn't care about the wall around it.
Checking an existing door frame
If a door already fitted is sticking, rattling, or showing uneven gaps around the door edge, the frame may have shifted. Use Spirit Level Online in Wall Mode to check each jamb for plumb. Even 1–2° of lean is enough to cause a door to bind or swing open on its own. Re-shimming and re-fixing the affected jamb usually resolves it without replacing the whole frame.
For all levelling jobs around the home — shelves, TV mounts, picture frames, and more — Spirit Level Online runs directly in your browser with no app download required.