How to Write a Corrugated Box Specification: 12-Field Template + 20 kg Worked Example

A corrugated box specification is a one-page document that fixes twelve things: internal dimensions, box style, ply, flute, the grammage and burst factor of every paper layer, minimum bursting strength, minimum edge crush, minimum box compression, moisture and Cobb limits, printing, joint and closure, and the stacking condition the box has to survive. Send those twelve fields to any box plant in India and you get comparable quotes and a carton that performs the same whoever makes it. Send "5-ply box, 500 × 400 × 300, good quality" and you get five different boxes at five different prices, none of them guaranteed to hold a stack through August.

This guide goes through the template field by field, gives the numbers behind each one, and finishes with a filled specification for a 20 kg engineering part travelling by road from Rajkot to Pune. It applies to any corrugated box, but the example is the kind we make most often: a plain 5-ply shipper with a foam fitment inside.

The 12-field template

Add three commercial lines under the table — quantity per lot, delivery schedule, and "sample to be approved before bulk production" — and the document is complete. Everything that follows explains where the numbers in the example column come from.

Fields 1–4: dimensions, style, ply and flute

Dimensions are internal, always, and written as length × width × height with length the larger of the two footprint sides and height the direction the flaps close across. A box plant adds board caliper when it cuts the blank; if you hand over external dimensions, the inside of a 5-ply box arrives about 14 mm smaller than you expected and the fitment will not go in. Work the internal size out from the product: product dimension + 2 × foam thickness + 3–5 mm of clearance on each axis, rounded up to the nearest 5 mm.

Style is a FEFCO code. The default is 0201, the regular slotted container, because it uses a rectangular blank with no waste and needs no die. If the box is much longer than it is wide, needs a double-thickness bottom, or has to sit on a retail shelf, another code fits better — the RSC guide covers the alternatives and the blank-size arithmetic.

Ply is chosen from product weight and stack height, not from how fragile the contents are. As a working rule, 3-ply carries up to about 10 kg on short domestic routes, 5-ply carries 10–25 kg and is the standard for export and engineering goods, and 7-ply carries 25–50 kg and above, often in place of a timber crate. Impact protection comes from the foam inside; ply decides whether the box keeps its shape under everything stacked on it.

Flute follows ply. In India a 3-ply shipper is normally B or C flute, a 5-ply board is BC, and E flute is reserved for printed retail cartons. Write the flute direction too: flutes must run vertically in the erected box, parallel to the height, or the stacking strength drops sharply. The flute types entry gives heights and characteristics; what matters on the specification is that the combination is named.

Field 5: paper — the line most purchase orders leave blank

Two boxes described as "5-ply" can differ by 30 % in paper weight and much more in strength. The only way to make them comparable is to write the GSM and burst factor of every layer, from the outer liner inward. The Indian trade shorthand is compact: a 3-ply board reads 150/120/150 with BF 18/16/18, meaning 150 GSM liners at 18 BF around a 120 GSM medium at 16 BF.

The reason both numbers are needed is that GSM is weight and BF is fibre quality. Each liner contributes roughly GSM × BF ÷ 1000 kg/cm² to the board's bursting strength: a 150 GSM liner at 18 BF gives about 2.7 kg/cm², a 180 GSM liner at 22 BF about 4.0 kg/cm², while a heavier-looking 180 GSM liner at 14 BF gives only 2.5 kg/cm². For export cartons ask for a virgin kraft outer liner of at least 20–22 BF; for domestic shippers semi-kraft at 16–18 BF is normal. The medium is where mills cut cost most quietly, and it is the medium that carries most of the edge crush, so do not leave it at "as per supplier".

Fields 6–8: bursting strength, ECT and BCT

These are three different tests answering three different questions, and a good specification carries all three.

Ranges are typical values for Indian kraft and semi-kraft boards; the actual figure depends on paper grade and box size. Bursting strength is what almost every Indian purchase order asks for, and it says nothing about stacking. The edge crush test is the figure that predicts whether a pallet of boxes survives 30 days in a warehouse, and box compression is the same question asked of the finished carton rather than a strip of board.

You can estimate BCT from ECT before a box exists using the McKee formula: BCT (N) ≈ 5.87 × ECT (N/m) × √(perimeter (m) × caliper (m)). For the example box, perimeter is 2 × (0.5 + 0.4) = 1.8 m, a BC board is about 6.5 mm thick, and ECT is 9 kN/m: 5.87 × 9,000 × √(1.8 × 0.0065) = 5.87 × 9,000 × 0.108 ≈ 5,700 N, or about 580 kgf. That is the laboratory figure for a fresh, dry box loaded evenly — the real box is derated by everything in field 12.

Fields 9–12: moisture, print, closure and the stacking condition

Moisture. Board should arrive at 7–9 % moisture. Every percentage point above that costs compression strength, and a box that sits in an unconditioned godown in Surat or Mumbai in August can climb past 12 % within days. Cobb60 — the grams of water 1 m² of liner absorbs in 60 seconds under IS 1060 — tells you how fast that happens. A sized virgin kraft liner typically returns 25–35 g/m²; unsized recycled liner can run well above 60. Specify a Cobb limit on the outer liner for monsoon dispatch and sea freight, and say nothing about it for a dry, one-week domestic route.

Printing. State the process (flexo up to 3 colours is standard; offset-laminated for retail), the colours, the artwork reference and which panels carry print. Keep solid heavy coverage off the flute area where possible — a fully flooded panel takes a small but measurable amount out of ECT because the print plate crushes the flute tips.

Joint and closure. The manufacturer's joint is glued as standard and stitched for heavy 7-ply. Closure is your operation, but the box has to suit it: a 48 mm BOPP centre seal on 3-ply and 5-ply, 72 mm on large 5-ply and 7-ply where the flap load is higher, or a strapping pattern for anything over 30 kg.

Stacking condition. This field converts the BCT into a pass or fail. State how many boxes high they will be stacked, for how long, in what humidity, and whether the pallet pattern is column or interlocked. Then work backward: load on the bottom box = (stack height − 1) × gross box weight, multiplied by a safety factor of 3 for dry, short storage and 4 or more for monsoon, container or interlocked stacking, which alone can cost 30–50 % of column-stack strength.

Worked example: a 20 kg gearbox housing, Rajkot to Pune

A cast-iron housing measuring 460 × 360 × 240 mm and weighing 20 kg is dispatched by road, about 650 km, and stored at the customer's plant for up to a month before use. It sits in 10 mm EPE end caps with EPE corner blocks, which adds 20 mm on each axis plus 5 mm clearance. The specification reads:

1. Size: 500 × 400 × 300 mm ID, ±3 mm (product 460 × 360 × 240 + 2 × 10 mm foam + clearance, rounded to 5 mm).
2. Style: FEFCO 0201 RSC.
3. Construction: 5-ply double wall — 20 kg sits comfortably in the 10–25 kg band.
4. Flute: BC, flutes vertical.
5. Paper: 180/22 (virgin kraft outer) – 120/16 – 120/16 – 120/16 – 180/22.
6. Bursting strength: ≥ 14 kg/cm².
7. ECT: ≥ 9 kN/m.
8. BCT: ≥ 350 kgf. Bottom box carries 4 × 21 kg = 84 kg; × 4 for monsoon storage = 336 kgf. The McKee estimate of about 580 kgf clears it with margin.
9. Moisture/Cobb: 7–9 %; Cobb60 ≤ 40 g/m² on the outer liner — the storage month may fall in monsoon.
10. Print: Flexo, 1 colour, part number and handling marks on two long panels.
11. Joint/closure: Glued joint; 48 mm BOPP centre seal plus two PP straps across the width.
12. Stacking: 5 high, column stacked on a 1,200 × 1,000 mm pallet, 30 days, unconditioned store.

Notice what the specification does not say: it does not call the housing "fragile" or ask for "heavy duty". Every line is a number the plant can test against, and every quote received against it is directly comparable.

Common mistakes on incoming purchase orders

• External dimensions written as size. The fitment does not go in. Always mark "ID".
• Ply without paper. "5-ply" alone lets the supplier choose the paper, and the lowest quote usually wins by using the lightest medium.
• Bursting strength only. The box passes the Mullen test and the pallet still collapses. Add ECT for anything stacked.
• "As per sample" with no retained sample. Six months later nobody can prove what was agreed. Keep a signed sample and a written spec.
• No stacking condition. Without stack height and storage period the plant cannot size the board and the buyer cannot judge a failure.
• No moisture limit for a monsoon dispatch. Board delivered at 12 % moisture has already lost a share of its strength before it is filled.
• Comparing quotes on different papers. A quote at 150/120/150 and one at 180/120/180 are not the same box; make suppliers quote against your paper line, not theirs.

Two decisions often made late belong in the specification: how the box is printed — see flexo vs offset printing on corrugated boxes — and whether the inner pack should be a duplex carton at all, covered in duplex box vs corrugated box.

How to send this to us

You do not need every field filled to get a quotation. Send us the product weight and dimensions, the route, the stack height and how long the boxes will be stored, and we will return a completed specification in this format with the paper, BS, ECT and BCT worked out for you — and a foam fitment designed to the same drawing. The packaging calculator gives a first-pass construction from weight and size, and our corrugated box page for Gujarat lists what we run in 3-ply, 5-ply and 7-ply, the dispatch lanes and the testing we do to IS 2771.