USPS Large Envelope Postage Calculator
Estimate First-Class Mail rates for flats based on current postal standards.
About USPS Large Envelope (Flat) Rates
Sending documents, manuscripts, or certificates often requires more than a standard letter stamp. The United States Postal Service (USPS) classifies these items as "Large Envelopes" or "Flats." This calculator helps you determine the necessary postage based on the weight of your mailpiece using current First-Class Mail Retail rates.
What Qualifies as a Large Envelope?
To qualify for the Flat rate rather than the Package rate (which is significantly more expensive), your mailpiece must meet specific physical standards regarding dimensions and flexibility.
| Dimension | Minimum | Maximum |
|---|---|---|
| Height | 6-1/8 inches | 12 inches |
| Length | 11-1/2 inches | 15 inches |
| Thickness | 1/4 inch | 3/4 inch |
Crucial Rule: The envelope must be flexible. If your large envelope is rigid (contains a hard object, cardboard stiffener that prevents bending, or a clasp), it may be classified as a "Non-machinable" Flat or even a Parcel, which incurs higher postage fees.
Understanding the Weight Limit
The First-Class Mail Large Envelope service is designed for items weighing up to 13 ounces.
- 0 to 1 oz: Base Rate (approx. $1.39)
- Additional Ounces: Increases by approx. $0.24 per ounce.
- Over 13 oz: The item automatically upgrades to Priority Mail. Priority Mail rates vary based on the destination zone and the specific weight, starting at a higher price point than First-Class Mail.
How to Measure Your Envelope
When measuring your envelope to ensure it fits the "Flat" criteria:
- Weigh it carefully: Use a kitchen scale or postal scale. Round up to the nearest ounce for calculation (e.g., 1.1 oz counts as 2 oz).
- Check flexibility: Ensure the envelope can bend around a standard mail sorting drum. Rigid items require parcel postage.
- Verify thickness: It must be uniformly thick. If one side is bulgy (over 3/4 inch), it becomes a parcel.
Frequently Asked Questions
Do I need a specific stamp?
You can use standard "Forever" stamps, but you will need multiple. For example, if the rate is $1.39 and a Forever stamp covers roughly $0.68, you would need to calculate the exact combination or purchase a specific "Additional Ounce" stamp or a global stamp at the post office.
What if my envelope is square?
Square envelopes often trigger non-machinable surcharges if they are letters, but for Large Envelopes, as long as they meet the minimum rectangular dimensions (6-1/8″ high by 11-1/2″ long), they typically qualify as Flats. However, verify with a postal clerk if the shape is unusual.