Foundations
Box spring sizes and foundation dimensions
A modern mattress foundation usually follows the mattress footprint, but current finished widths and lengths vary by roughly an inch and profile heights range from about 2 to 9 inches.
| Label or calculation | Dimensions | What it means |
|---|---|---|
| Twin | about 38 × 74–75 in (96.5 × 188–190.5 cm) | Finished length varies by current product line. |
| Twin XL | about 38–39 × 79.5–80 in (96.5–99.1 × 202–203.2 cm) | Also labeled Twin Long by some manufacturers. |
| Full | about 52.5–54 × 74–74.5 in (133.4–137.2 × 188–189.2 cm) | The largest current variation among these examples. |
| Queen | about 60 × 79.5–80 in (152.4 × 202–203.2 cm) | Usually one piece; split Queen products also exist. |
| King | two pieces, each about 38–39 × 79.5–80 in | The pair supports a nominal 76 × 80 in mattress. |
Use the mattress maker’s support and warranty instructions. “Box spring” is often used loosely for foundations with different constructions.
Footprint and profile are separate measurements
Width and length control whether the foundation fits the frame. Height controls the finished sleep-surface elevation once the mattress is added.
King foundations commonly arrive as two pieces
Current manufacturer examples divide King support into two Twin-XL-like pieces. Verify the pair, center support, and frame requirements rather than ordering one piece by label.
Measurement method
Start with the product and space you actually have.
- 1
Measure the frame opening. Record inside width, length, support ledges, and center support.
- 2
Check finished dimensions. Use the foundation specification, not only the size label.
- 3
Add vertical layers. Combine frame support height, foundation, mattress, pad, and topper.
- 4
Verify the support system. Follow the mattress and frame manufacturers’ current requirements.
Questions, answered
Frequently asked questions
How tall is a standard box spring?
Current foundation examples include roughly 9-inch standard profiles, 5–5.5-inch low profiles, and a 2-inch ultra-low option.
Should a box spring be exactly the mattress size?
It should support the intended mattress and fit the frame, but current finished dimensions can differ from nominal mattress dimensions by about an inch.