Stencil Area Ratio Calculator
The IPC-7525 area ratio for a stencil aperture is the aperture opening area divided by the aperture wall area, where the wall area is the aperture perimeter times the stencil thickness. A ratio at or above 0.66 is the accepted guideline for clean solder paste release. This calculator computes exactly that ratio for circular, square and rectangular apertures, from dimensions in millimetres, mils or inches, and tells you plainly whether the design passes or fails the 0.66 rule.
The verdict is the IPC-7525 guideline alone: 0.66 or above passes, below 0.66 fails. Paste type, board finish and printer settings still affect real print quality, so treat a pass as a design check, not a guarantee.
Quick Answer
A 0.5 mm circular aperture on a 0.12 mm stencil has an area ratio of 1.04, which passes the IPC-7525 guideline of 0.66: opening 0.196 mm squared, wall area 0.188 mm squared, ratio 0.196 / 0.188. That is the worked example preloaded in the tool. The old version of this page divided aperture area by component footprint instead, which is a different number that cannot be judged against the 0.66 rule at all.
Key takeaways
- IPC-7525 definition: area ratio = aperture opening area / aperture wall area, and wall area = aperture perimeter x stencil thickness.
- Circle shortcut: diameter / (4 x thickness). Rectangle: (length x width) / (2 x (length + width) x thickness).
- One verdict, no invented bands: 0.66 or above passes, below 0.66 fails. The closer a passing ratio sits to 0.66, the less margin it has for print variation.
- The ratio is per aperture: in an array, check the smallest aperture, since every identical aperture shares the same ratio and the count cancels out.
- Quick Answer
- How to use the stencil area ratio calculator
- What the IPC-7525 area ratio actually measures
- The IPC-7525 area ratio, and the 0.66 rule
- Worked example: 0.5 mm circle on a 0.12 mm stencil
- Reading the verdict: pass or fail at 0.66
- Arrays and multiple apertures: check the smallest one
- What competing calculators miss
- Frequently asked questions
- Add This Calculator to Your Website
- References
How to use the stencil area ratio calculator
Pick a shape, enter its dimensions and the stencil thickness in one unit, and click Check area ratio. Results stay hidden until you calculate, and Reset restores the worked example.
- Pick the aperture shape: circular, square or rectangular. Only the inputs for that shape are shown.
- Pick one length unit: millimetres, mils or inches. The unit applies to the aperture dimensions and the thickness alike, and everything converts to millimetres before the math runs.
- Enter the stencil thickness: the foil thickness, not the paste height. A thinner stencil raises the ratio and a thicker stencil lowers it, which is why thickness and aperture size must be checked together.
- Read the verdict: the opening area, the wall area, the ratio to four decimal places, the aspect ratio (aperture width divided by thickness) and the paste volume per aperture, with the full working shown underneath.
What the IPC-7525 area ratio actually measures
It measures whether solder paste will let go of the stencil walls cleanly. When the squeegee passes, paste fills the aperture; when the stencil lifts, the paste must release onto the pad instead of sticking to the walls. A large opening relative to its walls releases well. A small, deep opening has a large wall area relative to its opening, so surface tension holds the paste back and the joint prints short.
That is why the denominator is wall area, not footprint area. Footprint area describes how much of the board the component covers, which says nothing about release. Two apertures with the same footprint coverage can have wildly different wall areas if one is a fine circle on a thick stencil and the other is a wide rectangle on a thin one, and only the wall-based ratio separates them.
The old tool on this page made exactly that mistake: it divided total aperture area by component area and then graded the result against percentage bands, including a 0.66 line borrowed from the IPC rule. A coverage fraction and a release ratio share a number by coincidence only, so those bands have been removed with the old math.
The IPC-7525 area ratio, and the 0.66 rule
The definition, in full: area ratio = aperture opening area / (aperture perimeter x stencil thickness). The 0.66 figure is the widely accepted minimum for reliable paste release under IPC-7525: at or above it, release is expected to be clean; below it, insufficient paste and related defects become likely.
Circular aperture: area ratio = D / (4 x T), where D is diameter and T is stencil thickness.
Square aperture: area ratio = S / (4 x T), where S is side length.
Rectangular aperture: area ratio = (L x W) / (2 x (L + W) x T), where L is length and W is width.
Aspect ratio is the related but different check: aperture width divided by stencil thickness, with 1.5 often quoted as a minimum for the simplified case. A design can pass one and fail the other, so the tool prints both, but the verdict here is the area ratio alone, since that is what the keyword and the 0.66 rule describe.
Worked example: 0.5 mm circle on a 0.12 mm stencil
The worked example values are pre-filled above, so every number below is a real run of the calculator.
Run it yourself and the tool returns 1.0417 with a pass verdict, an aspect ratio of 4.17 (0.5 / 0.12) and a paste volume of 0.0236 cubic millimetres per aperture. The shortcut agrees: D / (4 x T) = 0.5 / 0.48 = 1.0417.
A failing case shows why thickness matters: the same 0.5 mm circle on a 0.25 mm stencil gives 0.5 / (4 x 0.25) = 0.50, a fail, even though the aperture never changed. The fix is a wider aperture, a thinner stencil, or both, followed by a recheck here. For a rectangular check, a 1.0 x 0.5 mm aperture on the same 0.12 mm stencil gives (1.0 x 0.5) / (2 x 1.5 x 0.12) = 0.5 / 0.36 = 1.3889, also a pass.
Reading the verdict: pass or fail at 0.66
The verdict is binary by design. At 0.66 or above, the design meets the IPC-7525 guideline for paste release. Below 0.66, it does not, and the honest response is to change the geometry, not to relabel the result. The old Excellent, Good, Acceptable and Poor percentage bands have been removed because they graded a different quantity and had no standing in the standard.
Margin still deserves a sentence. A ratio of 0.67 passes, but it passes by 0.01, so normal manufacturing variation in aperture size or foil thickness can push a borderline design over the line in production. A ratio comfortably above 0.66, like the worked example at 1.04, carries real headroom. Treat 0.66 as a floor, not a target.
Arrays and multiple apertures: check the smallest one
For an array of identical apertures, the area ratio is the same for every aperture, because opening and wall area both scale with the count and the count cancels out of the division. There is no separate array ratio to compute: check one aperture, and you have checked them all.
For a mixed design, check the smallest aperture, since it has the lowest ratio and fails first. A BGA array is the classic case: the ball apertures are usually the smallest openings on the board, so a pass on a large QFN aperture says nothing about the BGA. Enter the BGA aperture diameter and the stencil thickness here, and judge the design on that result. Pitch, the distance between aperture centres, does not enter the IPC ratio at all, which is why the old pitch-derived footprint shortcut has been removed.
Related ratio tools on this site cover neighbouring quantities without confusing them: a mark space ratio calculator for pulse timing, a gear ratio calculator for mechanical ratios, and an ohms to watts calculator for electrical power. None of them is a stencil check, and none of their ratios should be compared with 0.66.
What competing calculators miss
Credible stencil tools define the ratio the IPC way, which is why the old version of this page stood out: it computed aperture over footprint while presenting IPC bands, so its numbers could not be compared with any rival or with the standard. The rebuild removes that mismatch entirely, and drops the two features that existed only to serve the wrong definition: the component footprint input and the paste type selector, which labelled output without changing any computed number.
What remains is deliberately narrow: three shapes, one unit selector, thickness, and the ratio with its working. Rivals that bundle step stencils, aperture reduction percentages and paste volume optimisation still have their place for full stencil design, but a tool named for the area ratio should first get the area ratio right, and that is what this page now does.
Frequently asked questions
Aperture opening area divided by aperture wall area, where wall area is aperture perimeter times stencil thickness. It predicts whether solder paste releases cleanly from the stencil walls onto the pad.
The widely accepted IPC-7525 minimum for reliable paste release: an area ratio at or above 0.66 passes, below 0.66 fails. Treat 0.66 as a floor, not a target, since a ratio just above it has little margin for manufacturing variation.
Diameter divided by four times the stencil thickness: D / (4 x T). A 0.5 mm circle on a 0.12 mm stencil gives 0.5 / 0.48 = 1.0417, a pass.
(Length x width) / (2 x (length + width) x thickness). A 1.0 x 0.5 mm aperture on a 0.12 mm stencil gives 0.5 / 0.36 = 1.3889, a pass.
Because thickness sets the wall area directly: doubling thickness doubles wall area and halves the ratio for the same aperture. A fine aperture that passes on a thin stencil can fail on a thick one without changing size at all.
Widen the aperture, use a thinner stencil, or both, then recheck. Do not relabel the result: the ratio is a physical release check, and a failing aperture will print short paste in production.
Check one BGA aperture: enter its diameter and the stencil thickness. Identical apertures share the same ratio, so the count cancels out, and pitch does not enter the IPC ratio. Judge the design on its smallest aperture.
Area ratio is opening area divided by wall area, judged at 0.66. Aspect ratio is aperture width divided by stencil thickness, often quoted with a 1.5 minimum. They are different checks, and the tool prints both, but the verdict here is the area ratio.
The unit selector applies to every length on the page, so pick one unit for the whole calculation. If your aperture is specified in mils and your stencil in millimetres, convert one of them first (1 mil = 0.0254 mm) and then enter both in the same unit.
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References
- Indium Corporation. Stencil Printing for SMT Assembly Success Part 2: Print Metrics (area ratio as aperture area over wall area, AR = D/4t, the 0.66 acceptance rule). Accessed October 23, 2026.
- Prototype PCB Assembly. What is the area ratio for SMT stencils? (definition, worked 0.5 x 0.5 mm example, 0.66 to 1.5 acceptable range). Accessed October 23, 2026.
- Elepcb. PCB Stencil Guide: Materials, Thickness and SMT Printing Tips (area ratio formulas, 0.66 for leaded and 0.71 for lead-free processes). Accessed October 23, 2026.
- Gulyas, A. Paste Area Ratio Calculator (independent derivation of the aperture-over-wall formula, survey of the 0.66 guideline across manufacturers). Accessed October 23, 2026.
- JLCPCB. SMT Stencil Design Tips for Improving Solder Paste Deposition (area ratio = aperture area / (aperture wall height x aperture perimeter), keep above 0.66). Accessed October 23, 2026.
- PCBCart. Area Ratio Calculation in Solder Paste Stencil Design (area ratio as opening area over wall area, minimum 0.66 per IPC-7525). Accessed October 23, 2026.