Product requirements specification

A product requirements specification (also called a requirements specification, a product requirements document or PRD, or simply the spec) is a document that describes what a product must do, under which conditions and within which limits. At the end of development, the result is checked against it. It describes the problem and the requirements, not the design solution. German practice distinguishes the Lastenheft, the customer's statement of requirements, from the Pflichtenheft, in which the supplier describes how it will meet them.

A good specification usually covers the purpose of the product and its users, functional requirements, technical parameters with numerical values and limits (load, dimensions, weight, power, accuracy), the operating environment (temperature range, humidity, dust, vibration, IP rating), interfaces with surrounding parts and systems, the required service life and maintenance, and the applicable regulations and standards, for example for CE marking. It must also state the target manufacturing cost, the expected annual production volume and a list of required deliverables (models, drawings, bill of materials, prototypes).

Every requirement should be unambiguous, measurable and verifiable. Instead of "light and durable", the specification should say, for example, "weight no more than 1.2 kg, no damage after a 1 m drop onto concrete". Requirements are numbered and given a priority (mandatory, desirable, optional). For each one it is good practice to state how it will be verified: by calculation, test, measurement or document review. This creates the basis for verification and for acceptance of the result.

A specification differs from technical documentation in that it describes what the product must achieve, while the documentation describes how it is actually designed and manufactured. The specification is refined during development based on the results of prototypes and tests. At the right moment, usually before production tooling is ordered, it is frozen, and any further changes go through engineering change control.

Write the specification before you request quotes for development or design work. Without it, every supplier will offer something different and the quotes cannot be compared. It is also the basis of the development contract and the acceptance procedure, input for a design review and part of grant applications, where the project goals are assessed.

If you do not know the requirements precisely yet, start with a short version of what you do know and flag open questions explicitly. Clarifying them can be the first phase of the project, for example a feasibility study, a proof of concept or an analysis of competing products.

The most common mistake is a brief that describes a solution instead of a need ("a bracket welded from square tubing" instead of the required load capacity and installation space). It needlessly rules out better or cheaper options. The opposite problem is vague words without numbers, missing limit conditions (maximum load, minimum temperature) and a missing target cost and production volume. Volume in particular decides whether a part is worth 3D printing, machining or injection molding, and the design follows from that.

Also watch out for conflicting requirements without priorities and for a specification that changes only in emails. Record every change in a new revision of the document with the date and the reason, and agree it with the supplier, including the impact on price and schedule. At handover, the result is then verified against the latest approved version, not against what the participants remember.