Blister Pack Design Calculator
Cavity dimensions, card size, and formability for blister packs.
About this calculator
This calculator works out the thermoformed cavity and backing-card dimensions for a blister pack from the product's own size. It first adds a fixed 0.0625-inch clearance around the product on each side to get the cavity dimensions (a bit more clearance for depth, since it's only added once rather than doubled), then applies a 4-degree draft angle — standard practice in thermoforming so the formed plastic releases cleanly from the mold — to compute the wider opening at the top of the cavity versus its base. The backing card width scales with however many cavities you're placing side by side, each with its own sealing flange plus a bit of extra flange between cavities, and the card height adds a fixed 1.5 inches on top of the flanged cavity height to leave room for a hang-hole header. The most technically important output is the draw ratio — cavity depth divided by the smaller of the cavity's length or width — because thermoforming has a practical depth limit for a given opening size before the plastic thins too much in the corners to hold its shape reliably.
This calculator flags anything at or above 1.5 as not reliably formable for standard 15-mil PET film, which is a reasonable rule of thumb but not a universal one: material choice (PET vs. PVC), gauge, and mold sophistication can shift that threshold in either direction. Material cost is a rough per-square-inch estimate for film and card stock, useful for early comparisons but not a substitute for a supplier quote. Treat this as a first-pass sizing tool before final mold and artwork design.
Inputs
Results
Card Width (in)
3.27
Card Height (in)
5.77
How to Use This Calculator
- Enter product dimensions — length, width, and depth in inches — and the number of cavities per card.
- Enter the flange width (in), the seal margin around each cavity.
- Read the card width, card height, and total card area (sq in), plus the top opening length.
- Review the draw ratio — ratios of 1.5 or above are flagged as not formable for standard PET (see the Formable output).
- Use the card area and material cost per unit to estimate film and card material costs.
How the result changes with Product Width (in)
| Product Width (in) | Card Width (in) | Card Height (in) |
|---|---|---|
| 1 | 2.27 | 5.77 |
| 1.5 | 2.77 | 5.77 |
| 3 | 4.27 | 5.77 |
| 5 | 6.27 | 5.77 |
What each input means
- Product Length (in)
- Maximum product length.
- Product Width (in)
- Maximum product width.
- Product Depth (in)
- Maximum product height/depth.
- Cavities per Card
- Number of product cavities per blister card.
- Flange Width (in)
- Seal flange around each cavity.
What each result means
- Card Width (in)
- Overall backing card width.
- Card Height (in)
- Overall card height (including hang hole header).
- Card Area (sq in)
- Total card material area.
- Draw Ratio
- Cavity depth / opening. Should be <1.5 for standard PET.
- Formable? (1=Yes)
- Whether draw ratio is within thermoforming limits.
- Material Cost/Unit ($)
- Film + card material cost estimate.
- Top Opening Length (in)
- Cavity opening with draft angle.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersProduct Length (in) = 3, Product Width (in) = 2, Product Depth (in) = 1, Cavities per Card = 1 = 5 input(s) provided
- Calculate Card WidthCard Width = (topOpeningW + 2 * flangeWidthIn) * cavityCount + flangeWidthIn * (cavityCoun...3.27 = 3.27
- Calculate Card HeightCard Height = topOpeningL + 2 * flangeWidthIn + 1.55.77 = 5.77
- Calculate Card AreaCard Area = cardWidthIn * cardHeightIn18.9 = 18.9
- Calculate Draw RatioDraw Ratio = cavityDIn / min(cavityLIn, cavityWIn)0.5 = 0.5
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why is the depth clearance different from the length and width clearance?
Length and width each get clearance added on both sides of the product, so the calculator adds 2 × 0.0625 in to each of those dimensions. Depth clearance is only added once, since the product only needs room to fit down into the cavity from one direction — there's no second face on the depth axis needing its own clearance gap.
What does the draw ratio actually tell me, and why the 1.5 cutoff?
Draw ratio is cavity depth divided by the smaller of the cavity's length or width, and it estimates how much the plastic film has to stretch to form the cavity's corners and walls relative to its opening size. Above roughly 1.5 for standard 15-mil PET, the film tends to thin out too much in the corners during thermoforming to reliably hold its shape, which is why the calculator flags anything at or above that ratio as not formable.
How does adding more cavities change the card size?
Card width scales with cavity count directly: each cavity needs its own top opening plus flange width on both sides, and the calculator adds an extra flange width between each pair of adjacent cavities. Card height doesn't change with cavity count — it's based only on a single cavity's top-opening length plus flange and the fixed 1.5-inch header allowance, since cavities in this layout are arranged in a single row.
Why is 1.5 inches added to the card height beyond the cavity and flange?
That fixed 1.5-inch addition is reserved for a hang-hole header — the strip of card stock above the cavities used for a punched hole so the finished blister pack can hang on a peg hook in retail display. It's a flat allowance rather than something driven by product dimensions.
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