The case volume calculation says 42 L, so why does only 34 L fit?

Answer: The mouth multiplies to 42.0 L, corners, ribs, groove and draft remove 2.04 L, and the rectangular envelope cuts the rest to 34.2 L.

How much is lost by opening the corner radius from R10 to R25?

Answer: Only 0.02 L of volume, but the load gives up 30 mm of length and usable volume drops 3.2 L.

Do factories quote case volume calculation from the cavity or the outside?

Answer: Nearly all quote the mouth multiplied out, a few take outside volume times 0.65, a spread of 2 to 5 L.

Are ribs worth the volume they take?

Answer: Eight ribs cost 0.07 L but narrow clear width by 6 mm, so drop two when equipment sits within 6 mm of full width.

How do I put usable ratio into incoming inspection?

Answer: Require mouth dimensions, envelope and usable volume at ±1.0 mm and ±0.3 L, then sample three cases per lot.

Case Volume Calculation: Cavity, Corners, Usable Ratio

Quick answer: Case volume calculation starts at the cavity mouth, then deducts R10 corners, R25 fillets and draft, so a 42 L case yields 34.2 L of usable space.

Three volume figures and where the gap comes from
Measuring cavity length, width and depth
What corners and ribs subtract
Wall thickness, shrink and draft allowance
Usable ratio bands and sizing math
FAQ

1. Three volume figures and where the gap comes from

A case rated 42 L measures 500 × 362 × 232 mm across the cavity mouth, multiplying to 42.0 L. Loaded with boxy equipment the clear space is 480 × 362 × 197 mm, or 34.2 L, so 18.6% of the label disappears.

Case volume calculation needs three figures kept apart. The label multiplies the mouth and deducts nothing. The geometric cavity, 40.0 L, deducts corners, fillets, ribs and groove. The usable volume, 34.2 L, counts only what a rectangular load can occupy.

Only the third figure depends on load shape. A sharp corner must sit inside the radius, so length loses two radii while width loses nothing, cutting section area 4.0%.

Rated volume	Cavity mouth L×W×D	Geometric cavity	Rectangular envelope	Usable volume	Usable ratio
22 L	410×290×185 mm	22.0 L	390×290×149 mm	16.9 L	76.6%
42 L	500×362×232 mm	40.0 L	480×362×197 mm	34.2 L	81.4%
68 L	600×430×265 mm	65.2 L	580×430×225 mm	56.1 L	82.0%
95 L	670×480×295 mm	90.6 L	650×480×252 mm	78.6 L	82.8%
130 L	760×540×320 mm	125.2 L	740×540×272 mm	110.3 L	84.0%

The smallest case scores worst because fixed overhead scales against depth: 36 mm of boss, rib and step against 185 mm.

2. Measuring cavity length, width and depth

Measure the narrowest section: tape 10 mm below the rim, wall to wall, at both ends and mid-length, keeping the minimum. Depth runs from floor rib to rim, with the lid land measured separately.

Case volume calculation check: a black case open on grey, flat foam in the lid, nine base compartments with two blocks removed

Rim readings add 1.5 to 2.0 mm, worth 0.3 L. Ribs sit on one wall, so a mid-span reading reads 3 to 6 mm narrow, which the minimum rule folds in.

Lid depth stands alone: a 15 to 20 mm ring boss and a 6 to 9 mm gasket groove are structure, not space. Adding lid to base overstates the total by 1.2 to 1.8 L.

Measured item	Tool	Reading position	Tolerance
Cavity length	Tape, 1 mm graduations	10 mm below rim, minimum of 3	±1.0 mm
Cavity depth	Depth gauge	Floor rib top to rim	±1.5 mm
Corner radius	Radius gauge	All four corners	±1.0 mm
Wall thickness	Ultrasonic gauge	3 points per face, averaged	±0.3 mm

3. What corners and ribs subtract

Corners and ribs hit case volume calculation twice, and only once in the arithmetic. Each corner removes 0.2146 × R² × depth, so R10 at 232 mm deep costs 0.02 L for four corners, 0.05% of the label.

The geometry costs more than the volume. Clear length falls 20 mm and section area drops from 181 000 to 173 760 mm², a 4.0% loss. At R25 the load gives up 30 mm and usable volume falls 3.2 L.

Floor fillets multiply by perimeter: R25 on 1724 mm is 0.23 L. Eight ribs 4 mm wide, 6 mm high and 190 mm long take 0.07 L, yet each narrows clear width by 6 mm. Gasket groove and lid boss add 0.52 L, and neither can go without losing the IP rating.

Deduction	Formula	Volume removed on a 42 L case	Share of label
Four R10 corners	0.2146×R²×H×4	0.02 L	0.05%
R25 floor fillet	0.2146×R²×perimeter	0.23 L	0.55%
Gasket groove and lid boss	Groove area×perimeter + boss	0.52 L	1.24%
Eight side ribs	Width×height×length×count	0.07 L	0.17%
1.5° draft taper	Frustum volume difference	1.20 L	2.86%
Total	—	2.04 L	4.86%

4. Wall thickness, shrink and draft allowance

Draft is the largest deduction: 1.5° over 232 mm pulls each side in 6.1 mm, so the floor section shrinks to 487.8 × 349.8 mm and the cavity falls from 42.0 to 40.8 L.

Case volume calculation review: a man taking a phone selfie in a showroom, shelves of orange, yellow, blue and olive cases behind him

Wall thickness sets the per-side loss. At 7.0 mm instead of 5.5 mm the same outside dimensions cost 1.3 L, and recovering it means thinning the wall, which trades against stiffness.

Shrinkage is the missed layer. PP runs 1.5% to 2.0%, so a 500 mm direction comes out 8 to 10 mm short and the cavity sits 4.5% to 6.0% under the drawing. A quote built on mould dimensions hands that offset to the buyer.

Usable height assembles from mouth step 8 mm, lid boss 18 mm and floor rib 9 mm, leaving 197 mm of 232 mm. A four-point boss in place of a full ring recovers 9 to 11 mm.

5. Usable ratio bands and sizing math

Across more than 30 case types, usable ratio spans 72% to 85%: flat cases under 0.5 depth-to-width sit at 76% to 79%, mainstream shapes at 80% to 83%, deep cavities above 0.8 at 83% to 85%, and heavy ribbing at 72% to 75%.

Running case volume calculation backwards is more useful. 30 L of clear space at 81% needs a 37.0 L label, rounded up to 38 to 40 L; the same load in a 77% case needs 39.0 L, a full size higher. Three numbers belong in the purchase order: mouth dimensions, rectangular envelope, usable volume, at ±1.0 mm and ±0.3 L. A label reading about 42 L is not inspectable.

Conclusion

Case volume calculation reduces to three moves: read the mouth, deduct the structure, close the envelope around a rectangle. A 42 L case survives all three at 34.2 L, and 81.4% is its real capacity. Set thresholds accordingly: above 82% release the lot; between 78% and 82% check the load shape, pricing square equipment at the low end while round or soft loads can use the label; below 78% request the measured cavity drawing and envelope table, and price at 75% if neither arrives.

This article discusses container-side specifications and design implications only, and does not cover any handling, loading or use of the contents.