Ratio Decidendi vs Obiter Dicta — what they are and why it matters

Two Latin phrases dominate first-year UK law: ratio decidendi (“the reason for deciding”) and obiter dicta (“things said by the way”). Knowing which is which is the gateway skill of common-law analysis — you cannot apply precedent if you cannot find the ratio.

Ratio decidendi is the legal principle that determines the outcome. It is the rule the court applies to the material facts to reach its decision. The ratio binds future courts under the doctrine of precedent (stare decisis): a lower court must follow it; a court of equal rank usually will.

Obiter dictaare remarks the judge makes that are not strictly necessary to decide the case. Hypothetical examples, comments on counterfactuals, observations about how a different fact pattern might have been decided — all obiter. Obiter is persuasive only. It can become the ratio of a later case, but it doesn’t bind on its own.

How to find the ratio. Read the judgment with two questions in mind: what was actually decided, and what facts were essential to that decision? The ratio sits at the intersection. A useful rule of thumb: if you could remove a paragraph and the outcome of the case wouldn’t change, that paragraph is probably obiter.

Why it matters in essays. Examiners reward students who cite the ratio precisely. Saying “Donoghue v Stevenson [1932]established the modern law of negligence” is too vague. Better: “Lord Atkin’s neighbour principle — that one owes a duty of care to those one ought reasonably to have in contemplation as likely to be affected — is the ratio of Donoghue v Stevenson [1932] AC 562 (HL).” The first sentence paraphrases the case’s significance; the second nails the ratio.

Multiple judgments, multiple ratios. When several judges decide a case (e.g. five Law Lords in the House of Lords), each may write a separate speech with their own reasoning. The binding ratio is the principle the majority agreed on. Where the majority disagree on the reasoning but agree on the outcome, the case may have no clear ratio at all — this is where you cite carefully and explain the divergence.

Obiter that became ratio. Some of UK law’s most important rules began life as obiter and were elevated by later courts. Lord Denning’s remarks in Central London Property Trust v High Trees House [1947]on promissory estoppel were strictly obiter — the case was decided on a different point. Subsequent cases adopted the High Trees principle as their ratio, and it is now binding doctrine.

Distinguishing. The flip side of identifying ratios is distinguishing: showing that an apparently binding case turns on a material fact that isn’t present in your scenario. Distinguish, and the precedent doesn’t bind. Practise this on our case library — every brief flags facts the ratio depended on.

For more Latin and doctrinal terms in plain English, see our UK law glossary.