Introduction
A reply envelope is the return envelope you enclose with a bill, donation appeal, or survey, so the recipient can respond without addressing or stamping anything themselves. Its size is dictated by the outer envelope it nests inside.
The two standard reply envelope sizes are #9 (for #10 outers) and #6¾ (for compact statements). This guide covers the dimensions, the Business Reply Mail rules that govern reply envelopes, and how they pair with bills and remittances.
Standard Reply Envelope Sizes
Reply envelopes are sized to nest inside the outer envelope without folding.
| Size | Dimensions | Nests In |
|---|---|---|
| #9 | 3⅞” × 8⅞” | #10 |
| #6¾ | 3⅝” × 6½” | #10 or #6¾ outer |
| #7 | 3¾” × 6¾” | Small statements |
| #8⅝ | 3⅝” × 8⅝” | #10 |
The #9 is the most common reply envelope because it slips cleanly inside a #10 outer:
| Trait | #9 Reply Detail |
|---|---|
| Dimensions | 3⅞” × 8⅞” |
| Outer pairing | #10 |
| Fit | No fold needed |
| Use | Bills, appeals, surveys |
#9 vs #6¾
The choice between #9 and #6¾ comes down to the outer and the contents.
| Factor | #9 | #6¾ |
|---|---|---|
| Size | Larger | Smaller |
| Outer | #10 | #10 or smaller |
| Contents | Payment slip, check | Check, short form |
| Feel | Standard reply | Compact |
Use #9 when the reply includes a payment slip or fuller form; use #6¾ for a simple check-return.
| If the reply includes… | Choose |
|---|---|
| Full payment slip | #9 |
| Just a check | #6¾ |
| Survey form | #9 |
Business Reply Mail (BRM)
Business Reply Mail is the postal program that lets you pre-pay the reply postage.
| BRM Element | What It Means |
|---|---|
| Permit | Sender’s postal account |
| Pre-printed indicia | Postage pre-paid |
| Recipient pays nothing | You cover return cost |
| FIM/IMb | Sorting barcodes |
BRM makes reply frictionless for the customer, which lifts response:
| Benefit | Why It Matters |
|---|---|
| No stamp needed | Easier to respond |
| Higher response | Lower friction |
| Trackable | Barcode on piece |
Reply Envelope Layout
A reply envelope is pre-addressed and (with BRM) pre-stamped.
| Element | Placement |
|---|---|
| Return address | Front, your address |
| BRM indicia | Upper-right |
| FIM/IMb barcode | Upper edge |
| Window (optional) | For account number |
The layout must keep the barcode and indicia clear for automated sorting:
| Rule | Why |
|---|---|
| Clear indicia zone | Machine-read postage |
| Barcode unobstructed | Sorting |
| Return address legible | Delivery to you |
Our window envelope design standards guide covers window placement.
Reply Envelopes in Billing
Reply envelopes are the second half of a billing system.
| Billing setup | Reply envelope |
|---|---|
| Invoice + payment | #9 reply |
| Statement + remit | #9 or window |
| Donation appeal | #9 BRM |
| Survey | #9 BRM |
The outer #10 (or statement envelope) carries the bill out; the #9 reply carries the payment back. For the billing side, see our statement and remittance envelopes guide.
Conclusion
The standard reply envelope sizes are #9 (3⅞” × 8⅞”, for #10 outers) and #6¾ (3⅝” × 6½”, for compact statements). Pair #9 with full payment slips, #6¾ with simple check returns, and add Business Reply Mail to pre-pay postage and lift response. For the outer side, see our number 10 envelope standards guide.
Need reply envelopes for your billing or fundraising mail? Request a quote.