Seat load and whole-chair load are two different tests
A seat-load figure is measured with the load pressed onto the seat area. It tells you the seat panel or the seat rails will not collapse under a person. That is a useful number, and it is the only structural number most suppliers put on a quote, because it is the fastest one to produce.
A whole-chair load rating puts the load through the completed chair: seat and back assembly, frame, joints and legs, tested as one unit. Nothing is clamped, nothing is braced, and nothing is removed to make the test easier. The chair has to stand on its own four legs and hold the load without permanent deformation, cracking at a joint, or a leg splaying out of position.
The difference matters because of where chairs actually fail in a dining room. Seat panels rarely break. What fails is the joint between the back post and the seat rail, the corner block under the seat, or the leg where it meets the apron. None of those parts are loaded in a seat-only test, so a seat-load number can look excellent on a chair that will be loose within two seasons.
What the 150 kg figure is, and what it is not
The figure is a static load. The chair is loaded, held, inspected and released. It is not a fatigue test, so it does not by itself tell you how the chair behaves after ten thousand sittings, a guest tipping it back onto two legs, or a member of staff dragging it across a tiled floor with a full table beside it.
150 kg is a design margin, not a statement about the heaviest guest you should expect. It gives a working allowance above normal adult weight so that the chair is not running at its limit during ordinary service. A chair rated at exactly the weight of a large adult has no margin left for the dynamic load of someone sitting down quickly.
Static and fatigue are separate questions, and a serious specification asks both. A chair can pass a 150 kg static load and still develop movement at the joint after a year of heavy covers, because the failure mode is wear rather than fracture.
How to read the test report
A usable structural report names the model tested, the load applied, where the load was applied, how long it was held, and what counted as a failure. If the report only states a number with no loading point, it is a marketing figure rather than a test result.
Keep structural reports and material reports apart. A timber report confirming compliance with restricted substances, or a leather report confirming heavy-metal content by ICP-OES, says nothing about whether the chair holds weight. Both matter, but they answer different questions, and a supplier who offers one in place of the other is usually short of the one you asked for.
When you compare two suppliers, compare the same test. A whole-chair rating from one factory and a seat-load rating from another are not competing numbers, and choosing the larger of the two tells you nothing.
Questions to put in your RFQ
Ask whether the rating is seat-only or whole-chair, and ask for the loading point in writing. Ask how long the load is held and what the pass criterion is, because “no visible damage” and “no permanent deformation measured after release” are different standards. Ask whether the tested model is the model you are buying, in the same frame material and the same joint construction.
If your project uses metal restaurant chairs and wood restaurant chairs in the same room, ask for a report on each. The load path through a welded steel frame and through a mortise-and-tenon timber frame is not the same, and a result from one does not transfer to the other. Our own 150 kg whole-chair load test is run on the complete chair with the load applied to the seat and back assembly, and reports are available for the models in restaurant furniture products on request.
What this changes in your specification
Two changes are worth making on the drawing. First, state the test rather than the number: write “whole-chair static load, applied to seat and back assembly” so a supplier cannot answer with a seat-only figure. Second, tie the rating to the seat height you actually order. A chair built to 460 mm carries load through a different geometry from one built to 430 mm, and a custom seat height is a new structure even when the frame looks identical.
This is why custom dimensions should be confirmed on the drawing before production rather than agreed in a message. The seat height you need, the frame you have chosen and the load rating you are relying on are one decision, not three.
Two models that show how the two tests land in practice are the JP-C001 wood dining chair and the JP-C002 metal dining chair. Both are rated on the completed chair rather than on the seat alone, and both are quoted against your drawing rather than a stock size, so the rating you ask about is the rating of the chair that arrives.