Guide · Bags
How to size a frame bag or top-tube bag for your bike
Choose a frame bag by your frame's dimensions, not by liters, and count your bottles. How bolt-on and strapped top-tube bags differ, plus a measuring worksheet.
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Short answer
Choose a frame bag by your frame's dimensions, not by liters, and include your water bottles and anything already mounted in the triangle when you measure.[1] For a top-tube bag, check whether your bike has top-tube bolts; if not, you need a strapped bag.
Size by the frame, not by capacity
Apidura says frame pack size should be chosen by the dimensions of the frame, not by desired carrying capacity.[1] A bag with more liters is no help if it cannot fit inside your triangle, or if it blocks your bottle cages.
The frame triangle differs a lot between bikes and sizes. A small road bike frame and a large gravel bike frame can leave very different space, so measure your own bike rather than going by its type.
Count your bottles and fittings
Apidura's sizing tool asks riders to photograph the bike and to include water bottles and frame-mounted accessories in the photo, so clearance comes out accurate.[2] It also offers a printable A4 or US Letter template and a database by bike make and model.
That instruction is the useful part for any brand. A full-frame bag and two bottle cages compete for the same space. Decide first whether you want bottles inside the triangle, then size the bag around them.
Top-tube bags: bolt-on or strapped
There are two common ways to fix a top-tube bag:
- Bolt-on. Apidura's Expedition Bolt-On Top Tube Pack (1L) attaches with two bolts to top-tube bosses plus a strap around the steerer.[3]
- Strapped. Apidura directs bikes without top-tube bolts to its strapped version.[3]
Bolt spacing is not universal. Apidura's page gives no bolt spacing in its text and advises riders on smaller frames to check pack length.[3] Measure the distance between your own top-tube bolts, and the free top tube behind the stem, before you buy a bolt-on bag.
Do not drill a frame to add bolts. If your bike has no top-tube bosses, choose a strapped bag.
Picks that fit3 products
All three are strap-on top-tube bags for bikes without top-tube bolts. Topeak Fuel Tank: choose the Large size.
More on choosing
Topeak Tri DryBag: choose the Medium size, black. ROCKBROS 039BK: choose STYLE 1, black, 1 L. Measure the free top tube behind your stem and check each listing's dimensions against it. On Amazon, check that 'Ships from / Sold by' shows Amazon.com or the brand.
Smaller strapped top-tube bag
Saddle bags in brief
Large saddle bags hang off the seatpost and saddle rather than inside the frame. Restrap's 18-liter Saddle Bag attaches to the seatpost without mounts or screws and uses a holster with a separate dry bag.[4] Restrap states no maximum load, no tire or saddle clearance and no dropper-post compatibility on that page, so treat those as unknowns and check your seatpost exposure and rear-tire gap yourself.
Picks that fit2 products
Topeak BackLoader, 10 L, black: a seatpost bag that mounts to the seatpost and saddle rails.
More on choosing
Topeak BackLoader X, 15 L, green: a holster on the seatpost and saddle rails holding a removable bag; Topeak states more than 23 cm and 13 cm of clearance (its L1 and L2). For either, check that you have enough exposed seatpost and clearance above the rear tire, and ask the maker about dropper posts before you buy. On Amazon, check that 'Ships from / Sold by' shows Amazon.com or the brand.
Seatpost bag with a holster, larger size
Measuring worksheet
Print this or copy it into a note, and fill it in with the bike on its wheels.
| Measurement | Your bike |
|---|---|
| Inside length of top tube (head tube to seat tube) | ____ mm |
| Inside length of down tube | ____ mm |
| Inside length of seat tube | ____ mm |
| Bottle cages in the triangle (how many, and where) | ____ |
| Other fittings in the triangle (pump, cables, battery) | ____ |
| Top-tube bolts? Distance between them | Yes / No, ____ mm |
| Free top tube behind the stem | ____ mm |
| Exposed seatpost above the frame (for a saddle bag) | ____ mm |
| Gap from saddle rails to rear tire | ____ mm |
Compare those numbers with the bag maker's sizing tool or listed dimensions. If two sizes look close, the smaller one that clears your bottles is the safer choice.
Next
Questions
How do I measure my bike for a frame bag?
Measure the inside of the frame triangle along each tube and note where bottle cages and other fittings sit. Apidura says to choose frame pack size by the frame's dimensions, not by how much you want to carry, and offers a sizing tool and a printable template.[1][2]
Will a frame bag fit with water bottles?
Only if you size for them. Apidura's sizing instructions ask riders to include water bottles and frame-mounted accessories in the bike photo so clearance is accurate.[2]
Do I need bolts on my top tube for a top-tube bag?
Only for a bolt-on bag. Apidura's bolt-on top-tube pack uses two top-tube bolts plus a steerer strap, and Apidura directs bikes without those bolts to its strapped version.[3]
Is there a standard bolt spacing for top-tube bags?
We found no source that sets one. Apidura gives no bolt spacing in its text and advises riders on smaller frames to check pack length, so compare your own bolt positions with each bag's.[3]