Unistrut sizes come down to one dimension you need to get right and a part number that tells you the rest. The original section, P1000, is 1-5/8 in by 1-5/8 in in 12 gauge steel, and that 1-5/8 in channel opening is the size almost every fitting, nut and accessory is built around. Everything else in the range is a variation on it: doubled up, shallower, slotted differently, or in a different finish. This page covers the sizes, the section properties, what the suffix letters mean, and how much load a run will actually carry.
P1000: The Baseline Section
| Property | Value |
|---|---|
| Outside dimensions | 1-5/8 in × 1-5/8 in |
| Steel thickness | 12 gauge |
| Weight | 190 lb per 100 ft (1.90 lb/ft) |
| Area of section | 0.555 in² |
| Moment of inertia, axis 1-1 | 0.185 in⁴ |
| Moment of inertia, axis 2-2 | 0.236 in⁴ |
| Section modulus, axis 1-1 | 0.202 in³ |
| Section modulus, axis 2-2 | 0.290 in³ |
| Hole size and spacing | 9/16 in, on 2 in centers |
| Standard lengths | 10 ft and 20 ft |
Two things in that table are worth pausing on, because they explain how the channel behaves.
First, the weight is consistent with the area. Steel weighs about 0.2836 lb/in³, so 0.555 in² × 0.2836 × 12 = 1.89 lb/ft, which matches the published 1.90. Nothing surprising, but it confirms the section is solid 12 gauge with no hidden lightening.
Second, and more useful in practice: the two axes are not the same, and the weaker one is the one you would expect to be stronger. Axis 2-2 has the higher moment of inertia at 0.236 in⁴, and its section modulus of 0.290 in³ works out to exactly half the 1-5/8 in depth, which is what you get for a symmetric axis. Axis 1-1 is the open direction. Its section modulus of 0.202 in³ implies the extreme fiber sits about 0.92 in from the neutral axis rather than 0.81 in, because the open slot removes material from one face and drags the centroid toward the solid back.
The practical consequence: orient the channel with the slot facing away from the load where you can, so the solid back is in tension at the bottom of a span. Mounting a trapeze with the slot facing down is normal because that is where the fittings go, and the load tables account for it, but it is not the stiffest orientation.
How Much Load Will It Carry?
Span dominates everything. These are maximum allowable uniform loads for a single P1000 channel:
| Span | P1000 uniform load | P1001 (back-to-back) uniform load |
|---|---|---|
| 24 in | 1,690 lb | — |
| 48 in | 850 lb | — |
| 84 in | — | 5,170 lb |
| 120 in | 340 lb | 3,690 lb |
Read the pattern rather than the individual numbers. Doubling the span from 24 in to 48 in roughly halves the capacity, and going to 120 in cuts it to a fifth of the 24 in value. For uniform load on a simple span, capacity falls off in proportion to span, so a run twice as long carries half as much. Deflection behaves far worse than that, which is usually what actually limits a support for conduit or cable tray.
The P1001 column shows why back-to-back sections exist. At 120 in, P1001 carries 3,690 lb against P1000’s 340 lb, roughly eleven times as much from twice the steel. That is the depth effect: welding two channels back to back takes the section from 1-5/8 in deep to 3-1/4 in deep, and bending capacity scales with depth far faster than with area. It is the same reason a deeper I-beam beats a heavier shallow one, and the same principle behind a castellated beam.
The Part Numbers
| Part | Size | Description |
|---|---|---|
| P1000 | 1-5/8 × 1-5/8 | The standard section, solid back |
| P1000T | 1-5/8 × 1-5/8 | Slotted, 9/16 × 1-1/8 in slots |
| P1000SL | 1-5/8 × 1-5/8 | Long slots |
| P1000H3 | 1-5/8 × 1-5/8 | Round holes on all three closed sides |
| P1001 | 1-5/8 × 3-1/4 | Back-to-back, solid |
| P1001T | 1-5/8 × 3-1/4 | Back-to-back, slotted |
| P1001C | 1-5/8 × 3-1/4 | Back-to-back, double combination |
The P1001 back-to-back sections give the 3-1/4 in depth in the table above. Section area for P1001 is 1.111 in², which is almost exactly twice the 0.555 in² of a single channel, as you would expect. Its moments of inertia are 0.928 in⁴ about axis 1-1 and 0.471 in⁴ about axis 2-2, so joining two channels back to back multiplies the strong-axis stiffness by about five while only doubling the weak axis.
Finishes
| Suffix | Finish | Where it belongs |
|---|---|---|
| PG | Pre-galvanized zinc | The usual indoor default |
| HG | Hot-dipped galvanized | Outdoors, damp, corrosive environments |
| GR | Perma-Green III | Coated, higher corrosion resistance |
| ZD | Perma-Gold | Coated alternative |
| PL | Plain | To be painted, or dry interior only |
| EA | Extruded aluminum | Light weight, non-magnetic |
| SS / ST | Stainless steel | Wet, washdown, chemical exposure |
Pre-galvanized and hot-dipped are not interchangeable outdoors. Pre-galvanized steel is coated as coil before the section is formed, so the cut ends and the punched holes are bare steel. Hot-dipped galvanizing happens after fabrication, so the coating covers the cut edges too. Anywhere it will get wet, specify HG or stainless. This is the same distinction covered in does galvanized steel rust.
One thing to watch on aluminum: the EA sections share the dimensions but not the strength or the load tables. Do not carry a steel load figure across to an aluminum channel, and avoid pairing aluminum channel with plain steel fittings in wet conditions, where the dissimilar metals will drive galvanic corrosion.
Where the Load Tables Stop Applying
Published uniform load figures assume a simple span carrying a distributed load, with the channel restrained from twisting. Three common field situations fall outside that.
Point loads. A single heavy item hung at midspan produces twice the bending moment of the same total weight spread along the channel. Halve the tabulated uniform capacity as a starting point for a midspan point load, then check deflection separately.
Seismic and vibration. Bracing for seismic load is a designed system with its own requirements, not a matter of adding another piece of strut. Runs supporting rotating equipment need the fittings checked for loosening, since a channel nut relies on friction and serration rather than a through-bolt.
The anchor, not the channel. In most failures the strut is not what gives way. The capacity of the concrete anchor or the connection into the structure above is usually the governing number, and it is often far below the channel’s own rating. See anchor bolts for how those are sized and what reduces their capacity.
Unistrut Sizes FAQ
Is Unistrut the same as strut channel?
Unistrut is a brand name that became the generic term, like Sheetrock for gypsum board. Strut channel from other manufacturers uses the same 1-5/8 in profile and is generally dimensionally compatible, but load ratings and steel thickness are the manufacturer’s own. Do not mix load tables between brands.
What size is standard Unistrut?
1-5/8 in by 1-5/8 in, 12 gauge, which is the P1000. If someone says “standard strut” without qualifying it, this is what they mean.
What is the difference between P1000 and P1001?
P1001 is two P1000 channels joined back to back, giving a 1-5/8 × 3-1/4 in section. Twice the steel, but roughly five times the strong-axis stiffness because of the extra depth.
What size threaded rod fits the holes?
The 9/16 in holes in P1000 suit 1/2 in rod and hardware. Some back-to-back slotted sections use larger slots sized for 5/8 in rod, so check the specific part rather than assuming.
Can I use it as a structural beam?
No. Strut channel is a support system for services and equipment. Anything carrying building load belongs to a designed structural member, sized by an engineer. See types of steel beams for what those are.
Sources: Unistrut P1000 channel specification and elements of section for dimensions, area, moments of inertia, section moduli and allowable uniform loads, and the P1000 product notes for weight and finish designations. Consistency of weight against area was checked at 0.2836 lb/in³ for steel. Load figures are the manufacturer’s; always use the current published tables for the exact part and finish. Last updated: September 2026.
This page is reference information only and is not engineering advice. Support systems for piping, equipment and seismic bracing must be designed for the specific installation. Full disclaimer.