# Certainty and completeness of contractual terms

> **Key takeaway:** Terms must be certain/complete enough to enforce. Courts uphold commercial bargains (Hillas). Bare agreements to agree generally fail (Walford; May & Butcher). Machinery/formula or performance may save the deal (RTS). Construction ≠ voidness.

- **Jurisdiction:** England & Wales
- **Practice area:** Commercial
- **Last reviewed:** 2026-08-04
- **Interactive page:** https://kttclegal.info/library/notes/Commercial/certainty-and-completeness
- **Keywords:** certainty, completeness, agreement to agree, Walford v Miles, Hillas v Arcos, RTS Flexible Systems, vague terms, formation of contract

## What is this about?

An agreement that is too vague or incomplete will not be enforced as a contract. English law distinguishes mere difficulty of interpretation from true uncertainty, and will often uphold commercial deals where the parties clearly intended to be bound and left workable machinery for filling gaps.

## What is the core rule?

The parties must agree terms that are sufficiently certain and complete for the court to enforce. A contract is not void for uncertainty merely because it is difficult to construe; the court seeks to give effect to the parties' intention (Hillas v Arcos). Agreements to agree on essential terms are generally unenforceable (Walford v Miles), but an agreement may be saved where a workable objective standard, formula, or third-party determination is provided, or where the parties have already performed on a common understanding (RTS Flexible Systems).

## What are the elements or test?

1. Did the parties intend to create legal relations and conclude a bargain, or only negotiate?
2. Are the alleged essential terms identified (price, subject matter, duration, etc.)?
3. Can those terms be given meaning by construction, implication, custom, or an agreed mechanism (valuation, arbitration, standard form)?
4. Is the clause an unenforceable agreement to agree, or a binding obligation with incomplete but curable detail?
5. Has subsequent performance shown a concluded contract despite missing formal documentation?

## Which authorities matter?

- **Hillas & Co Ltd v Arcos Ltd (1932) 147 LT 503** — Courts strive to uphold commercial bargains and will not defeat them for avoidable uncertainty of language.
- **Walford v Miles [1992] 2 AC 128** — An agreement to negotiate in good faith is generally too uncertain to enforce as a lock-out/negotiation obligation of that kind.
- **May and Butcher Ltd v R [1934] 2 KB 17n** — Classic illustration that price left entirely to future agreement without mechanism can leave no contract.
- **RTS Flexible Systems Ltd v Molkerei Alois Müller GmbH & Co KG [2010] UKSC 14, [2010] 1 WLR 753** — Supreme Court: parties may be bound despite incomplete formalities where intention and performance support a concluded contract.

## How does this apply in practice?

This note addresses certainty/completeness of terms, not consideration or intention as free-standing formation elements (see companion notes). Statutory gap-filling (e.g. CRA/SGA implied terms) may supply content once a contract exists. Always identify what the parties treated as essential.

## What are common pitfalls?

- Treating every open commercial term as fatal uncertainty without looking for a workable formula
- Confusing difficult construction with legal uncertainty
- Assuming 'subject to contract' always means no deal after substantial performance (see RTS)
- Pleading Walford as if it voids every duty of good faith in a wider negotiated framework without reading the actual clause

## When would a practitioner use this?

Use when challenging or defending incomplete heads of terms, MOUs, price-to-be-agreed clauses, and long-term supply frameworks.

## Quick reference

Terms must be certain/complete enough to enforce. Courts uphold commercial bargains (Hillas). Bare agreements to agree generally fail (Walford; May & Butcher). Machinery/formula or performance may save the deal (RTS). Construction ≠ voidness.

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*Reference material from [KTTC Legal](https://kttclegal.info/), not legal advice. Work product supports instructing solicitors and barristers under their supervision. England & Wales.*