Nonmachinable letters: the rules that change what you pay

The letter size envelope of dimensions

Under the USPS domestic mail standards, a letter size piece must be at least 5 in long, 3.5 in high and 0.007 in thick. It must not exceed 11.5 in long, 6.125 in high or 0.25 in thick, and it must be rectangular with square corners. Weight for a letter is capped at 3.5 ounces.

Go beyond those maximums and the piece becomes a flat, which is a different and more expensive category, up to its own limits of 15 in long, 12 in high and 0.75 in thick, and which must stay flexible and uniformly thick.

Those numbers explain several familiar design facts. A No. 10 envelope at 4.125 x 9.5 in sits comfortably inside them. A 6 x 9 in envelope squeezes in only because 6 in is just under the 6.125 in height limit. A 9 x 12 in envelope is a flat, not a letter.

The aspect ratio rule

The rule that catches designers out is shape. A letter size piece is nonmachinable if its aspect ratio, length divided by height, is less than 1.3 or more than 2.5.

That range is narrower than it looks. Work it out for anything you are considering.

Every square envelope fails, because a square has a ratio of exactly 1. That is why square invitations carry a surcharge, and it is a design decision worth making with eyes open rather than discovering at the counter.

The other nonmachinable triggers

Shape is one cause. The standards list several others, and they are the ones that bite on invitations and small gift mailings.

• Rigidity. A piece that will not flex through the equipment is nonmachinable, which includes very thick card and anything with a stiff insert.
• Clasps, strings, buttons or similar closures.
• Uneven thickness, such as a coin, a pen, a bead or a raised wax seal, which makes the piece lumpy.
• Plastic wrapping on the outside.
• Address orientation that does not match the way equipment expects to read the piece.
• Failing a minimum thickness where the piece is at the larger end of letter dimensions.

That list is why so many wedding suites end up nonmachinable. A square envelope, a thick cotton card, a ribbon and a wax seal is four triggers in one envelope.

Designing to stay machinable

If keeping postage low matters, design the whole suite against the rules from the beginning rather than trying to rescue it later.

Choose a rectangular envelope with a ratio between 1.3 and 2.5. An A7 at 1.38 and a No. 10 at 2.30 both sit safely inside. Keep the total thickness under 0.25 in, including every enclosure. Keep the piece flexible; if the card is stiff enough that you cannot bend the sealed envelope gently, expect a surcharge.

Put nothing lumpy inside. Loose confetti, seed paper beads, charms, pressed flowers and raised seals all create uneven thickness. Keep decorative seals to flat stickers, and put anything three dimensional in a parcel rather than a letter.

Set the address block the way equipment expects to read it, parallel to the longest edge. The postal addressing standards set out the format for the delivery address, and following them removes a whole class of avoidable failures.

When nonmachinable is the right choice anyway

Sometimes the design is the point. A square invitation with a wax seal looks like nothing else in the mailbox, and for 120 wedding guests the extra handling cost is small against the effect.

The mistake is not choosing nonmachinable. The mistake is not knowing. Decide deliberately, budget for it, and take a fully assembled and sealed sample to a Post Office before you order the full quantity so nobody is surprised.

For delicate pieces, ask about hand canceling. Real wax seals crack under machine pressure, which is why hand canceling is recommended for them: it means the postmark is applied by hand rather than by equipment. It is a request rather than a guaranteed service, so design for the possibility that the piece still goes through a machine.

Machinability checklist

• Keep the piece rectangular with square corners.
• Stay within 5 to 11.5 in long, 3.5 to 6.125 in high and 0.25 in thick.
• Divide length by height and keep the result between 1.3 and 2.5.
• Accept that every square envelope is nonmachinable.
• Keep the sealed envelope flexible, with no stiff inserts.
• Put nothing lumpy or three dimensional inside a letter.
• Avoid clasps, strings and outer plastic wrapping.
• Set the address parallel to the longest edge, following postal addressing standards.
• Take a sealed, fully assembled sample to a Post Office before ordering the full run.