An SOP describes a process: what triggers it, who is involved, the sequence of steps across those roles, and what happens at each decision point. A work instruction describes a single task in enough detail that one person can perform it correctly at their station, including tolerances, settings, and the specific tools.
Put simply: the SOP says the Quality Inspector performs the final inspection. The work instruction tells that inspector exactly how, with which gauge, against which tolerance.
The practical difference
- Scope. An SOP spans multiple roles. A work instruction covers one person doing one task.
- Level of detail. An SOP names the step. A work instruction specifies how to execute it, down to values and settings.
- Decision points. SOPs contain branches, because processes fork. Work instructions are usually linear, because a task at a station either succeeds or is escalated.
- Audience. An SOP is read by anyone who needs to understand the process. A work instruction is read at the point of work by the person doing it.
- Change frequency. Work instructions change when tooling, materials, or drawings change, which is often. SOPs change when the process itself changes, which is rarer.
Which one do you need?
Write an SOP when the question is "how does this get done here, and by whom". Write a work instruction when the question is "how do I do this specific thing correctly".
Most teams need both, connected: the SOP references the work instruction at the step where it applies. That reference is also what an ISO 9001 auditor looks for, because it demonstrates the document hierarchy actually exists rather than being asserted.
The mistake worth avoiding
Writing one document that tries to be both. It ends up too general to follow at the bench and too detailed to read as a process, so the operator ignores it and the manager cannot use it to explain the workflow. If your document has a torque value in it and also describes a handoff between two departments, it is two documents.