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Three numbers decide which cargo box fits a Subaru Outback: the box’s length, its weight, and how much of your roof rating is left after it. Cubic feet matter least. An Outback has the longest roof of the three Subaru models, so most boxes physically fit. The real limits are the hatch swing and the roof weight rating.
Quick Picks
- Best for most Outback owners: Thule Motion 3 L: 16 cu ft (450 L), about 77 in long, around 44 lb empty.
- Best alternative brand: Yakima SkyBox 18: 18 cu ft, same size class as the Motion 3 XL.
- Best for families: Thule Motion 3 XL: 18 cu ft (500 L), about 85 in long, around 51 lb.
- Best for long trips: Thule Motion 3 XXL: 21 cu ft (600 L), about 91 in long. Run the hatch test below first.
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Box Sizes Compared
Dimensions are the manufacturers’ published exterior sizes. Weights are retailer-listed and vary by a few pounds between sources, so treat them as close, not exact.
| Box | Length x width x height (in) | Capacity | Empty weight | Hatch test (Yakima) |
|---|---|---|---|---|
| Thule Motion 3 L | about 76-78 x 34-35 x 16.5 | 16 cu ft (450 L) | about 44 lb | Not published |
| Thule Motion 3 XL | 84.8 x 34.1 x 16.9 | 18 cu ft (500 L) | about 51 lb | Not published |
| Thule Motion 3 XL Low | about 85 x 35 x 14 | 14 cu ft (400 L) | about 53 lb | Not published |
| Thule Motion 3 XXL | 91.3 x 36.2 x 18.1 | 21 cu ft (600 L) | 50-57 lb (sources differ) | Not published |
| Yakima SkyBox NX 16 | 75.5 x 35 x 16 | 16 cu ft | 48 lb | 47 in |
| Yakima SkyBox NX 18 | 84 x 36 x 16.5 | 18 cu ft | 53 lb | 50 in |
| Yakima SkyBox NX XXL | 90.5 x 36 x 19 | Not listed | 60.5 lb | 55 in |
Sources: Thule Motion 3 spec sheet and retailer listings; Yakima SkyBox instruction sheets. An older Carbonite 18 is 92 x 36 x 16 in, 52 lb, and needs 57 in.
The Hatch Test: Measure Before You Buy
Length decides whether the Outback’s hatch hits the box. Yakima publishes a simple test: with the hatch fully open, measure from the center of the front crossbar to the edge of the open hatch. If your number is at least the figure in the table (47 in for an NX 16, 50 in for an NX 18, 55 in for an NXXL), the box fits with its rear edge at the bars. Thule does not publish an equivalent number, so for a Thule box measure the same distance and check that the box’s rear edge can sit at or ahead of the rear crossbar.
A box that is too far back is the usual cause of hatch trouble, and sliding it forward inside its mounting range fixes it. See How Far Forward Should a Cargo Box Sit? for the positioning rule.
How Much Weight Is Left on Your Roof
Subaru’s roof rating covers everything up there: aftermarket bars if you fitted them, the box, and the gear inside. Subtract the box first. What remains is your real packing budget.
| Outback | Roof rating (driving) | Left with a 44 lb Motion 3 L | Left with a 51 lb Motion 3 XL | Left with a 57 lb XXL |
|---|---|---|---|---|
| 2018-2025 (not Wilderness) | 150 lb | about 106 lb | about 99 lb | about 93 lb |
| 2022-2025 Wilderness | 200 lb | about 156 lb | about 149 lb | about 143 lb |
| 2026 (all trims) | 220 lb | about 176 lb | about 169 lb | about 163 lb |
That is why an XXL on a 150 lb Outback is a poor trade: you gain 5 cubic feet and lose about 13 lb of gear capacity compared with the L. Full tables by year and trim are in Subaru Roof Weight Limits by Model and Year.
What a Box Adds to the Outback’s Own Cargo Space
Behind the rear seats, a 2020-2025 Outback holds about 32.6 cu ft, and 75.7 cu ft with the seats folded. A 16 cu ft box adds roughly half of the behind-the-seats space, and an 18 cu ft box adds more than half. The 2026 Outback is a little roomier inside (34.6 cu ft behind the seats), and the 2018-2019 models hold 35.5.
| Outback years | Behind rear seats | Seats folded | Roof rating (driving) | Height to top of rails |
|---|---|---|---|---|
| 2018-2019 | 35.5 cu ft | 73.3 cu ft | 150 lb | 66.1 in |
| 2020-2025 | 32.5-32.6 cu ft | 75.6-75.7 cu ft | 150 lb (Wilderness 2022-25: 200 lb) | 66.1 in (Wilderness 66.9) |
| 2026 | 34.6 cu ft | 80.5 cu ft | 220 lb | 67.5 in (Wilderness 68.3) |
Source: Subaru trim comparison specifications, 2018-2026. If you only need an extra 10 cu ft for a trip or two a year, the L is enough. Go to an XL when you regularly fill the cargo area and still have gear left over.
Which Size to Pick
- Two people, weekend trips: a 14-16 cu ft box (Motion 3 L or SkyBox NX 16) holds duffels and a tent without hatch issues.
- Family of four, ski or camping trips: an 18 cu ft box (Motion 3 XL or SkyBox 18). Check the hatch test and keep the packed weight under your roof budget.
- Skis or long gear: the XL and XXL boxes are the ones long enough for skis. Retailer listings give the XL a 200 cm ski limit and the XXL 215 cm.
- Garage with low clearance: the Motion 3 XL Low is 14 in tall against 16.5 in for the L. See the garage clearance guide for the height math.
Mistakes Outback Owners Make
- Buying by cubic feet. Two 18 cu ft boxes can differ in length by several inches. Length decides hatch clearance.
- Forgetting the box’s own weight. A 57 lb XXL uses more than a third of a 150 lb rating before you pack anything.
- Ignoring the trim. The 2022-2025 Wilderness is rated 200 lb driving, the same-year standard Outback 150 lb.
- Leaving it on all year. It adds drag and wind noise even when empty.
FAQs
What size cargo box fits a Subaru Outback?
Most boxes from 14 to 21 cu ft physically fit the Outback’s roof. The practical limit is the hatch swing (check with the Yakima test above) and your roof weight rating, not the roof length.
Can an Outback fit a 21 cu ft box?
Yes, a Thule Motion 3 XXL (about 91 in long) can fit if the hatch test passes and you stay under the roof rating. On a 150 lb Outback, the 50 to 57 lb box leaves about 93 to 100 lb for gear.
Does the Outback Wilderness fit the same box sizes?
Yes, the box sizes are the same, and the Wilderness has a higher rating (200 lb driving for 2022-2025). Its roof is about 0.8 in taller, which matters for garages and loading.
Does a larger box hurt gas mileage?
Yes. Larger boxes add drag, most noticeably at highway speed. See Do Cargo Boxes Affect MPG?
Final Thoughts
For most Outback owners the Thule Motion 3 L or an 18 cu ft XL is the right call. Measure the hatch distance, subtract the box weight from your roof rating, and pick the largest box that passes both checks.
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