The Blind Size Calculator calculates correct blind sizes from window recess dimensions, factoring brackets, fitting type, overlaps, and operating clearance.
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What Is a Blind Size Calculator?
A blind size calculator is a practical measuring and planning tool. It converts your window opening measurements into the “order size” for roller, venetian, vertical, Roman, and cellular blinds. It includes allowances for hardware, fabric or slat thickness, clearances, and overlaps so the finished blind fits the space and functions smoothly.
In construction and fit-out work, small errors turn into rework, delays, and wasted materials. The calculator standardizes deductions for inside mounts and overlaps for outside mounts. It also estimates stack or roll-up height, cloth width versus overall hardware width, and approximate wastage for cutting. You leave with a clear, auditable set of dimensions to send to suppliers or your workshop.
The Mechanics Behind Blind Size
Blind sizing is about more than the visible panel. The headrail or tube, end caps, brackets, and control mechanism all consume space. Inside mounts need clearance so the blind operates without scraping the reveal. Outside mounts need enough overlap to block light and cover trim.
- Inside mount requires side-to-side and depth clearance, accounting for out-of-square walls and bracket projection.
- Outside mount relies on top, bottom, and side overlap to block light and hide trim or gaps.
- Roller blinds have an overall width wider than the cloth due to brackets and control hardware.
- Venetian and vertical blinds form a stack when raised or drawn; stack height or width must clear handles and sills.
- Materials expand, contract, or stretch; timber slats and some fabrics vary with humidity and temperature.
Good sizing balances function and finish. Provide enough clearance for smooth operation, but not so much that light leaks or gaps look sloppy. The calculator encodes these mechanics so you apply consistent rules to every opening.
Blind Size Formulas & Derivations
These equations drive the calculator’s results. They convert raw width and height into order sizes that include allowances for fit, overlap, and hardware. Typical industry ranges are shown where brand-specific data varies.
- Inside mount width: OrderWidth = OpeningWidth − FitAllowance. Common FitAllowance: 6–12 mm to avoid binding.
- Inside mount drop: OrderDrop = OpeningHeight − SillClearance. Typical SillClearance: 5–10 mm to prevent rubbing.
- Outside mount width: OrderWidth = OpeningWidth + 2 × SideOverlap. SideOverlap often 30–75 mm per side for light control.
- Outside mount drop: OrderDrop = OpeningHeight + TopOverlap + BottomOverlap. Add more overlap for blackout.
- Roller cloth width: ClothWidth = OrderWidth − ClothDeduction. ClothDeduction typically 25–35 mm for brackets and controls.
- Venetian stack height (approx.): Stack = HeadrailHeight + SlatCount × SlatThickness × PackingFactor. PackingFactor ≈ 1.1–1.3.
Manufacturers publish exact deductions, tube diameters, and bracket sizes. When you know those values, plug them in for precise results. If not, use typical ranges and add a small safety margin to reduce risk of rework and wastage.
Inputs and Assumptions for Blind Size
The calculator asks for a few key inputs and applies standardized assumptions so results are consistent across rooms and installers. You can override defaults if your hardware specs differ.
- Opening width and height: Measure in three places each; use the tightest number for inside mounts.
- Mount type: Inside mount (reveal) or outside mount (face or architrave).
- Overlaps and clearances: Side, top, and bottom overlap; sill and handle clearance.
- Product and material: Roller, venetian, vertical, Roman, or cellular; slat size or fabric thickness.
- Hardware data: Headrail height or tube diameter; bracket projection; cloth deduction if known.
- Wastage allowance: Extra material for cutting, trimming, and site tolerances.
Openings that are out-of-square, very large, or near obstacles need special care. Adjust overlaps and clearances for radiators, tiles, handles, or deep mullions. For damp rooms or timber slats, increase allowances to account for movement. Always check minimum recess depth for your chosen mechanism.
Using the Blind Size Calculator: A Walkthrough
Here’s a concise overview before we dive into the key points:
- Measure window width in three spots (top, middle, bottom) and record the smallest value.
- Measure window height in three spots (left, center, right) and record the smallest value.
- Select mount type and set overlaps or fit allowances appropriate to your light and privacy needs.
- Choose the product type and enter hardware data (headrail height, tube diameter, cloth deduction) if available.
- Enter material details such as slat size or fabric thickness to estimate stack or roll diameter.
- Review the calculated order width and drop, plus notes on cloth width, stack, and expected wastage.
These points provide quick orientation—use them alongside the full explanations in this page.
Worked Examples
Example 1: Inside-mount roller blind in a modern reveal. Opening width is 1200 mm; height is 1350 mm. You set FitAllowance to 10 mm and SillClearance to 8 mm. The Calculator gives OrderWidth = 1200 − 10 = 1190 mm and OrderDrop = 1350 − 8 = 1342 mm. With a typical ClothDeduction of 30 mm, ClothWidth is 1190 − 30 = 1160 mm. If the selected fabric thickness is 0.4 mm and tube diameter is 38 mm, the approximate full-roll diameter is computed for installation clearance. What this means: order a 1190 × 1342 mm roller with cloth about 1160 mm wide, and confirm bracket clearances for a 38 mm tube.
Example 2: Outside-mount faux wood venetian with 50 mm slats. Opening width is 900 mm; height is 1200 mm. You want 50 mm overlap per side, plus 80 mm top and 20 mm bottom to cover trim and light gaps. The Calculator gives OrderWidth = 900 + 2 × 50 = 1000 mm. OrderDrop = 1200 + 80 + 20 = 1300 mm. Using SlatPitch of 52 mm, SlatCount ≈ floor(1300 ÷ 52) = 25. With a headrail height of 65 mm, slat thickness 2.8 mm, and PackingFactor 1.2, Stack ≈ 65 + 25 × 2.8 × 1.2 ≈ 149 mm, which clears the handle. What this means: order a 1000 × 1300 mm venetian; expect roughly 150 mm stack at the top when raised.
Accuracy & Limitations
Calculator outputs are only as good as your inputs and assumptions. Brands vary in hardware sizes and deductions. Buildings are rarely perfect. Allow for real-world variance, and confirm critical dimensions with product data sheets.
- Manufacturer deductions and bracket sizes differ by model; confirm before ordering in bulk.
- Fabric stretch, humidity, and thermal movement change actual drop and stack over time.
- Out-of-square openings need larger fit allowances to avoid rubbing and light leaks.
- Very wide blinds may deflect; consider center supports or multiple panels to reduce sag.
- Safety devices add depth or spacing; include them in recess depth checks.
Take test measurements, use a steel tape, and document every assumption. For high-spec projects, build a mock-up and validate clearances on site. A small cushion in allowances can reduce wastage and costly refabrication.
Units & Conversions
Blinds are sold and installed worldwide, so you often switch between metric and imperial. Consistent units keep dimensions, overlaps, and materials clear. Use these conversions to translate measurements before entering them in the Calculator.
| From | To | Conversion |
|---|---|---|
| 1 in | mm | 25.4 |
| 1 mm | in | 0.03937 |
| 1 cm | in | 0.3937 |
| 1 ft | mm | 304.8 |
| 1 oz/yd² | g/m² | 33.91 |
Multiply your measurement by the conversion value to switch units. For example, 47 in × 25.4 = 1193.8 mm. Use consistent units across all inputs, especially when combining hardware from different suppliers.
Common Issues & Fixes
Most sizing problems come from rushing measurements or missing an obstacle. You can prevent errors with a quick checklist and disciplined inputs.
- Window out of square: Use the smallest width and height; increase fit allowance.
- Handles or radiators clash: Increase top or side overlap, or choose a shallower bracket.
- Light gaps on roller blinds: Switch to reverse roll, increase outside overlap, or add side channels.
- Sag on wide spans: Split into two blinds on one headrail or add a center support.
- Fabric too narrow for tube: Confirm cloth deduction and choose a wider cloth or a smaller bracket set.
Before ordering, review a scaled sketch with dimensions and materials. Confirm all bracket projections and headrail heights. A five-minute cross-check saves days of rework and reduces wastage.
FAQ about Blind Size Calculator
Should I measure inside the trim or include the trim?
For inside mounts, measure inside the reveal and use the smallest width and height. For outside mounts, include the trim and add your planned side and vertical overlaps.
How much overlap should I use for blackout blinds?
Use at least 50–75 mm per side and 80–100 mm at the top and bottom. Add side channels if you need near-total darkness or have bright exterior light.
Can I cover a wide opening with one blind?
You can, but check sag and weight limits. Very wide blinds may bow or roll unevenly. Consider two blinds on one headrail or a linked system to reduce deflection and improve handling.
What if my window isn’t square?
Measure in three places for width and height and use the smallest values. Increase fit allowances for inside mounts, or switch to an outside mount with more overlap to hide irregularities.
Glossary for Blind Size
Inside Mount
A blind installed within the window reveal, requiring clearance on width and depth to operate without rubbing.
Outside Mount
A blind installed on the wall or trim, overlapping the opening to block light and cover gaps.
Headrail
The top housing that holds brackets, tube, or mechanisms. Its height and depth affect recess fit and stack.
Cloth Deduction
The reduction from the overall roller width to fabric width to allow for brackets and controls.
Stack Height
The compacted height of a venetian or Roman blind when fully raised, including headrail and packed slats or folds.
Overlap
The additional coverage beyond the opening on an outside mount, set on the sides, top, and bottom.
Recess Depth
The available depth inside the reveal. It must exceed bracket projection and allow operation without obstruction.
Wastage
The extra material consumed during cutting, trimming, and fitting, used to ensure clean edges and accurate sizing.
Sources & Further Reading
Here’s a concise overview before we dive into the key points:
- Blinds.com measuring guides for inside and outside mount
- Hunter Douglas measuring and installation resources
- British Blind and Shutter Association: Make it Safe guidance
- Window Covering Manufacturers Association: Safety information and standards
- Engineering Toolbox: Roll length and diameter relationships
These points provide quick orientation—use them alongside the full explanations in this page.