The Short Answer

Measure the opening width inside the brickmould at the top, middle, and bottom. Record all three and use the narrowest width when comparing the opening with Larson’s current size guide. Measure height from the threshold surface where the storm door will sit to the underside of the top mounting surface. Then compare both diagonal measurements to evaluate squareness.

For Larson’s current universal-hinged installation example, the diagonals should be within 1/8 inch of each other, and the mounting surface should provide one inch of width by one inch of depth. Those values apply to the referenced installation guidance; always verify the requirements for the exact model before ordering.

What Are You Actually Measuring?

The storm door normally attaches to the exterior mounting surface formed by the brickmould or exterior trim around the primary entry door. You are measuring the opening where the storm-door hinge rail, latch rail, drip cap, and bottom expander will operate.

You are not measuring:

• The width or height of the primary entry-door slab.
• The outside dimensions of an existing storm-door frame.
• The rough opening hidden behind the primary door jamb.
• The decorative casing outside the storm-door mounting location.
• A nominal size printed on an old invoice without field verification.

Measurement Summary

Tools for a Reliable Field Measure

• A readable tape measure with an undamaged hook.
• A level long enough to reveal meaningful trim or threshold conditions.
• A square or straightedge for localized mounting-surface checks.
• A flashlight for inspecting lower trim and concealed deterioration.
• A camera or phone for overall and detail photos.
• A field sheet that forces entry of all three widths and both diagonals.
• Pencil, removable tape, or labels for identifying photo locations.

Do not let a phone note containing one number become the order record. A structured worksheet exposes missing information before the contractor leaves the property.

Step 1: Photograph the Entire Entrance

Take straight-on exterior and interior photos before measuring. Include the landing, threshold, brickmould, primary door, hardware, porch ceiling, columns, railings, lights, cameras, address plaques, and nearby walls.

Photos serve three purposes:

1. They preserve the measurement context.
2. They reveal swing and hardware conflicts that numbers may not explain.
3. They allow the supplier and installer to ask better questions before ordering.

Step 2: Identify the Storm-Door Mounting Plane

Find the flat trim face where the hinge rail, latch rail, and drip cap will attach. Confirm that the same plane continues around the opening. Decorative trim, aluminum cladding, replacement wrap, masonry returns, and built-out jambs can create steps or recesses that interfere with the rails.

Larson’s universal-hinged overview calls for a clean, smooth exterior trim surface and says damaged trim should be repaired before installation. Inspect for:

• Rot, softness, splitting, loose joints, or weak attachment.
• Peeling paint, heavy caulk, old screw holes, or uneven wrap.
• Mounting faces that taper, twist, or do not align.
• Insufficient width or depth for the selected system.
• Hidden masonry, metal, or voids that affect fastening.

If the mounting surface cannot securely support the new door, repair the opening before installation. Do not treat a longer screw as a universal solution.

Step 3: Measure Width at the Top

Place the tape inside the left and right brickmould edges at the top of the mounting opening. Keep the tape level and measure between the surfaces where the storm-door rails will sit. Record the full measurement; do not immediately round it to a nominal door size.

Photograph the tape in position when the opening is unusual or the number is close to a size limit. A photo should support the field sheet, not replace it.

Step 4: Measure Width at the Middle

Repeat the measurement at approximately mid-height. This value can reveal bowed, twisted, or uneven trim that a top-and-bottom measurement misses.

If the middle is narrower, investigate why. Paint buildup or a localized trim defect may be correctable. A bowed or unstable mounting surface may require repair. Do not simply select the narrowest number and ignore the physical cause.

Step 5: Measure Width at the Bottom

Measure between the inside brickmould edges near the bottom, above the threshold where the rail extenders will terminate. Watch for flared jamb legs, thick sealant, threshold returns, sloped sill details, or trim damage.

Larson instructs users to use the narrowest of the top, middle, and bottom widths when matching the opening to its size guide. Record all three so the installer can see how much the opening varies.

Step 6: Measure Mounting Height

Measure from the threshold surface where the storm-door expander and sweep will operate to the underside of the top brickmould or head mounting surface. Do not automatically measure from the porch floor, interior floor, or primary-door sill cap.

If the threshold slopes or the head is not level, take height measurements at the left, center, and right. Record the actual mounting points and photograph unusual conditions. The bottom expander offers adjustment within product limits; it does not correct every height variation or threshold defect.

Step 7: Check Both Diagonals

Measure from the lower-left mounting corner to the upper-right corner. Then measure from the lower-right corner to the upper-left. Equal diagonals indicate a square rectangle; different measurements show the opening is out of square.

Larson’s current universal-hinged example says the diagonals should be within 1/8 inch of each other. If the difference exceeds the applicable product guidance, do not assume the rails can absorb it. Determine whether shimming is permitted, trim can be corrected, or a different ordering and installation strategy is required.

Step 8: Verify Mounting-Surface Width and Depth

Measure the flat face width and depth available for the rails on both sides and across the head. Larson’s referenced universal-hinged guidance calls for a mounting surface one inch wide by one inch deep.

The measurement must represent usable material—not caulk, thin cladding over a void, crumbling wood, or decorative projection. Confirm what the screws will enter and whether the surface is securely connected.

Step 9: Inspect the Threshold and Sweep Path

The bottom expander and sweep must close against the intended threshold area without dragging excessively or leaving an uncontrolled gap. Examine slope, steps, nosing, door sill projection, mats, weatherstripping, ice exposure, and drainage.

Document the vertical drop from the mounting opening to the porch or landing. A storm door must be able to swing without catching a rising surface, thick mat, stair nosing, or seasonal ice buildup.

Step 10: Check Handle and Closer Clearance

Measure the projection and position of the primary door handle, deadbolt, keypad, and decorative hardware. Compare them with the planned storm-door handleset location. The two handles must not collide when both doors close.

Also inspect room for the closer and brackets. Look for deep trim, glass, blinds, lighting, cameras, mail slots, doorbells, or other accessories. A correctly sized storm door can still be incompatible with the entry hardware.

Step 11: Determine Handing and Swing

For a universal-hinged model, determine which side will receive the storm-door hinges when viewed from the exterior. Larson notes that storm-door hinges are typically on the same side as the primary door hinges, but approach, traffic flow, handle interference, railings, walls, and other conditions can affect the decision.

Walk the approach as though carrying groceries. Open the primary door and visualize the storm-door swing. Check:

• Porch columns, railings, walls, lights, and landscaping.
• Landing depth and top stair position.
• Entry-door and storm-door handle interaction.
• Wind exposure that could catch the open storm door.
• Whether the selected model is universal or factory handed.

Larson Standard-Size Reference

Larson’s current online measuring guide lists these common opening ranges. Verify the chart again for the exact product and purchase date before ordering.

Larson also lists selected shorter and taller ranges, and offers custom sizes for various products. Do not infer custom availability from the standard chart; confirm it for the chosen model.

When the Opening Is Between Sizes

Do not round to the nearest nominal size. Compare the exact narrowest width and mounting height with the approved range for the selected model. Larson’s universal-hinged overview notes that hinge-side shimming may be used in certain conditions to reduce the opening, but the shim thickness and method must follow the applicable instructions.

If the opening falls outside the standard range, options may include:

• Permitted continuous shimming or mounting-surface build-out.
• Repairing or replacing brickmould to create a correct opening.
• Selecting another compatible Larson model.
• Ordering an available custom size.

Any opening modification changes the final measurement. Measure again after the repair before releasing the order.

Measuring an Existing Storm Door Replacement

Do not copy the old door size without measuring the current opening. The existing unit may have been incorrectly sized, heavily shimmed, installed over damaged trim, or altered in the field.

1. Record the existing brand, model, label, handing, and visible dimensions.
2. Measure the actual mounting opening independently.
3. Inspect concealed areas visible around old rails and closer brackets.
4. Plan removal, abandoned-hole sealing, trim repair, and disposal.
5. Confirm final measurements after any required repair.

Required Photo Set

• Full exterior elevation from the normal approach.
• Full interior view with the primary door closed.
• Top, middle, and bottom width measurements.
• Height measurement at the mounting plane.
• Both diagonal measurement positions.
• Left, right, and head mounting-surface profiles.
• Threshold, sill, sweep path, landing, and top stair.
• Entry handleset, deadbolt, keypad, and likely storm-handle zone.
• Closer area, lights, cameras, railings, walls, and swing obstructions.
• Any rot, cracks, wrap, old holes, sealant buildup, or repair areas.

Contractor Order-Release Checklist

1. Exact Larson model and current installation document identified.
2. Top, middle, and bottom widths recorded.
3. Mounting height verified at all relevant points.
4. Both diagonals recorded and squareness evaluated.
5. Mounting width, depth, material, and condition confirmed.
6. Threshold and expander-sweep relationship documented.
7. Handing, approach, swing, handle, and closer clearances approved.
8. Opening repairs completed or precisely included in scope.
9. Standard or custom size confirmed against current product guidance.
10. Field sheet, photos, product configuration, and customer approval retained.

Common Storm-Door Measuring Mistakes

• Measuring the entry-door slab instead of the storm-door mounting opening.
• Taking only one width measurement.
• Measuring height from the porch floor instead of the threshold mounting point.
• Ignoring an out-of-square opening because the narrowest width fits a chart.
• Counting wrapped, rotten, or loose trim as usable mounting material.
• Ordering before checking handle and closer clearance.
• Assuming the old storm door was correctly sized.
• Rounding dimensions to a nominal size too early.
• Failing to remeasure after brickmould repair or build-out.
• Keeping no photos or written record of who measured.

The Bottom Line

To measure for a Larson storm door, record three opening widths inside the brickmould, mounting height, both diagonals, and the usable mounting-surface width and depth. Then evaluate the threshold, handing, swing, entry hardware, closer space, trim condition, and obstructions.

For Davenport and Bettendorf contractors, a complete field sheet and photo record is the cheapest protection against a wrong-size order. Measure the real mounting opening, use the exact current Larson documentation, and stop the order when the surface needs repair.

Frequently Asked Questions