diff --git a/plans/rank.go b/plans/rank.go
new file mode 100644
index 0000000..6c7ba41
--- /dev/null
+++ b/plans/rank.go
@@ -0,0 +1,57 @@
+// rank.go — totally-ordered rank of plan tiers, shared across api/, worker/,
+// and any future module that needs to classify a tier transition as an
+// upgrade vs a downgrade.
+//
+// Two package-private rank functions used to live in the api repo
+// (internal/handlers/billing.go::tierRank and
+// internal/handlers/admin_customers.go::adminTierRank). They had subtly
+// different orderings — billing.go covered 6 tiers (anonymous .. team),
+// admin_customers.go covered 4 (free .. team) and was off-by-one against
+// billing for the same names. The discrepancy never bit production because
+// the admin surface never sees anonymous/growth, but it's a footgun waiting
+// to happen the moment the admin surface is widened.
+//
+// This file promotes a single canonical ordering. Callers compare ranks
+// (a.rank < b.rank ⇒ a is "lower tier") and MUST guard against the -1
+// sentinel returned for unknown tiers.
+
+package plans
+
+import "strings"
+
+// Rank returns a totally-ordered integer rank for the given plan tier name.
+// Higher rank = more capacity. The canonical ordering is:
+//
+//	anonymous = 0
+//	free      = 1
+//	hobby     = 2
+//	growth    = 3
+//	pro       = 4
+//	team      = 5
+//
+// Unknown tiers return -1. Callers that compare ranks to classify a
+// transition (upgrade vs downgrade vs renewal) MUST treat -1 as the
+// "no transition direction" verdict — i.e. emit no audit row rather than
+// guess which way an unknown tier sits relative to a known one.
+//
+// The function is intentionally case- and whitespace-insensitive so callers
+// don't need to pre-normalise. The "*_yearly" billing variants are NOT
+// special-cased here — pass them through CanonicalTier first if you want
+// "pro_yearly" to rank the same as "pro" (billing.go does exactly this).
+func Rank(tier string) int {
+	switch strings.ToLower(strings.TrimSpace(tier)) {
+	case "anonymous":
+		return 0
+	case "free":
+		return 1
+	case "hobby":
+		return 2
+	case "growth":
+		return 3
+	case "pro":
+		return 4
+	case "team":
+		return 5
+	}
+	return -1
+}
diff --git a/plans/rank_test.go b/plans/rank_test.go
new file mode 100644
index 0000000..8a4f13b
--- /dev/null
+++ b/plans/rank_test.go
@@ -0,0 +1,86 @@
+package plans_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"instant.dev/common/plans"
+)
+
+// TestRank_AllStandardTiers asserts the canonical ordering documented in
+// rank.go. Lock-in test — changing any of these values is an API break
+// because callers compare ranks across modules (api/, worker/).
+func TestRank_AllStandardTiers(t *testing.T) {
+	cases := map[string]int{
+		"anonymous": 0,
+		"free":      1,
+		"hobby":     2,
+		"growth":    3,
+		"pro":       4,
+		"team":      5,
+	}
+	for tier, want := range cases {
+		t.Run(tier, func(t *testing.T) {
+			assert.Equal(t, want, plans.Rank(tier),
+				"Rank(%q) — canonical ordering must remain stable", tier)
+		})
+	}
+}
+
+// TestRank_UnknownReturnsMinusOne covers the sentinel contract: any tier
+// name not in the canonical list returns -1 so callers can short-circuit
+// rather than guess a direction.
+func TestRank_UnknownReturnsMinusOne(t *testing.T) {
+	cases := []string{
+		"",
+		"enterprise",
+		"premium",
+		"basic",
+		"unknown",
+		"pro_yearly", // Yearly variants must be normalised via CanonicalTier first.
+		"hobby_yearly",
+		"   ", // Empty-after-trim stays unknown.
+		"pro-yearly",
+		"freetier",
+	}
+	for _, tier := range cases {
+		t.Run(tier, func(t *testing.T) {
+			if tier == "pro_yearly" || tier == "hobby_yearly" {
+				// Yearly variants intentionally return -1 — callers MUST
+				// pass them through CanonicalTier first. This row asserts
+				// the "don't auto-normalise" contract.
+				assert.Equal(t, -1, plans.Rank(tier),
+					"Rank(%q) — yearly variants must NOT auto-normalise (callers use CanonicalTier)", tier)
+				return
+			}
+			assert.Equal(t, -1, plans.Rank(tier),
+				"Rank(%q) — unknown tier must return -1 sentinel", tier)
+		})
+	}
+}
+
+// TestRank_MonotonicallyIncreasing asserts that the canonical chain
+// anonymous < free < hobby < growth < pro < team is strictly increasing.
+// This is the property callers actually depend on (a.rank < b.rank ⇒
+// a is the lower tier); the absolute values in TestRank_AllStandardTiers
+// could in principle be remapped, but the relative ordering can't.
+func TestRank_MonotonicallyIncreasing(t *testing.T) {
+	chain := []string{"anonymous", "free", "hobby", "growth", "pro", "team"}
+	for i := 1; i < len(chain); i++ {
+		prev := plans.Rank(chain[i-1])
+		curr := plans.Rank(chain[i])
+		assert.Less(t, prev, curr,
+			"Rank(%q)=%d must be strictly less than Rank(%q)=%d",
+			chain[i-1], prev, chain[i], curr)
+	}
+}
+
+// TestRank_CaseInsensitive covers the documented case-insensitive
+// behaviour — callers shouldn't need to normalise before calling.
+func TestRank_CaseInsensitive(t *testing.T) {
+	assert.Equal(t, 4, plans.Rank("PRO"))
+	assert.Equal(t, 4, plans.Rank("Pro"))
+	assert.Equal(t, 4, plans.Rank("pRo"))
+	assert.Equal(t, 2, plans.Rank("  hobby  "))
+}