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+---
+epic: 2
+story: 2.5
+title: "Natural Language Unit Expressions"
+status: draft
+---
+
+## Epic 2 — Unit Conversion System
+**Goal:** Support 200+ units across all major categories with SI prefix support.
+
+### Story 2.5: Natural Language Unit Expressions
+
+As a CalcPad user,
+I want to write unit conversions using natural language keywords like "in", "to", and "as" with plural and singular unit names,
+So that I can express conversions the way I naturally think about them.
+
+**Acceptance Criteria:**
+
+**Given** the expression `5 inches in cm`
+**When** the engine evaluates it
+**Then** it recognizes `in` as a conversion operator and `cm` as the target unit
+**And** the result is `12.7 cm`
+
+**Given** the expression `100 ft to meters`
+**When** the engine evaluates it
+**Then** it recognizes `to` as a conversion operator
+**And** the result is `30.48 meters`
+
+**Given** the expression `72 kg as pounds`
+**When** the engine evaluates it
+**Then** it recognizes `as` as a conversion operator
+**And** the result is approximately `158.733 pounds`
+
+**Given** the expression `1 foot in inches`
+**When** the engine evaluates it
+**Then** it recognizes both the singular `foot` and plural `inches`
+**And** the result is `12 inches`
+
+**Given** the expression `5 lbs in kilograms`
+**When** the engine evaluates it
+**Then** it recognizes the abbreviation `lbs` and full name `kilograms`
+**And** the result is approximately `2.26796 kilograms`
+
+**Given** the word `in` used in a context that is not a unit conversion (e.g., `5 in + 3 in`)
+**When** the engine evaluates it
+**Then** it correctly interprets `in` as the unit "inches" rather than a conversion operator
+**And** the result is `8 in`