THE NUMBERS, EXPLAINED
How to use this calculator.
Simple envelope fit allowance: add the insert thickness once to each planar dimension, plus clearance on both edges. This is an estimated flat-envelope starting size, not proof of fit; seam shape, stiffness and gussets require a physical sample.
Start with your inputs
Enter folded insert width (mm), folded insert height (mm), insert thickness (mm), clearance on each edge (mm). Keep amounts in the units shown on each field. The example is a starting point, not a recommended value.
| Input | Example value |
|---|---|
| Folded insert width (mm) | 148 |
| Folded insert height (mm) | 210 |
| Insert thickness (mm) | 2 |
| Clearance on EACH edge (mm) | 3 |
A worked example
With the example inputs above, estimated internal envelope width (mm) is 156. Change the inputs to explore your own scenario.
| Result | Example answer |
|---|---|
| Estimated internal envelope width (mm) | 156 |
| Estimated internal envelope height (mm) | 218 |
How this is calculated
Estimated internal envelope width (mm)
Insert width + thickness + two edge clearances.
Estimated internal envelope height (mm)
Insert height + thickness + two edge clearances.
Symbols and units
- a
- Folded insert width (mm)
- b
- Folded insert height (mm)
- c
- Insert thickness (mm)
- d
- Clearance on EACH edge (mm)
Use the units shown. A list keeps the order in which you enter its values. The symbol ^ means “raised to the power of”.
Assumptions and limits
Simple envelope fit allowance: add the insert thickness once to each planar dimension, plus clearance on both edges. This is an estimated flat-envelope starting size, not proof of fit; seam shape, stiffness and gussets require a physical sample.
Calculations use the values you enter. Blank or invalid inputs show an error instead of silently substituting zero. Results are displayed to useful precision; calculations retain the underlying numeric precision.
Questions about this tool
What does the result tell me?
Estimated internal envelope width (mm); Estimated internal envelope height (mm). The result applies to the method and inputs described above. Check the units and assumptions before using it in a decision.
Why might another calculator differ?
Differences in units, rounding, timing or model definitions can produce different answers. Compare the input definitions and the method, rather than only the final number. For this tool: Simple envelope fit allowance: add the insert thickness once to each planar dimension, plus clearance on both edges. This is an estimated flat-envelope starting size, not proof of fit; seam shape, stiffness and gussets require a physical sample.
Are my inputs stored or sent to AI?
Calculations on this page happen in your browser. The separate AI question finder sends its submitted conversation to OpenAI; these calculator fields are not sent. Favourites and recent tool names are stored locally. A shared link contains the inputs you choose to include.
Further reading
OpenStax: geometric formulas ↗
Background reading; this does not imply endorsement or independent review of SumFizz.
Review status
Software example and browser checks are recorded in our quality report. No external expert review is claimed. Created with coding assistance; calculations run as ordinary browser code.
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