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+---
+epic: 2
+story: 2.2
+title: "SI Prefix Support"
+status: draft
+---
+
+## Epic 2 — Unit Conversion System
+**Goal:** Support 200+ units across all major categories with SI prefix support.
+
+### Story 2.2: SI Prefix Support
+
+As a CalcPad user,
+I want SI prefixes (nano through tera) recognized on any applicable unit in both short and long form,
+So that I can naturally write expressions like `5 km` or `200 MB` without needing separate unit entries for every prefixed variant.
+
+**Acceptance Criteria:**
+
+**Given** the expression `5 km in miles`
+**When** the engine evaluates it
+**Then** it recognizes `km` as `kilo` + `meter`
+**And** the result is approximately `3.10686 miles`
+
+**Given** the expression `200 MB in GB`
+**When** the engine evaluates it
+**Then** it recognizes `MB` as `mega` + `byte` and `GB` as `giga` + `byte`
+**And** the result is `0.2 GB`
+
+**Given** the expression `3 µs in ms`
+**When** the engine evaluates it
+**Then** it recognizes `µs` as `micro` + `second` and `ms` as `milli` + `second`
+**And** the result is `0.003 ms`
+
+**Given** the expression `5 kilometers in miles`
+**When** the engine evaluates it
+**Then** it recognizes the long-form prefix `kilo` combined with `meters`
+**And** the result matches the short-form `5 km in miles`
+
+**Given** SI prefixes from nano (10^-9) through tera (10^12)
+**When** each prefix is applied to a compatible base unit
+**Then** the conversion factor is correctly applied: nano=10^-9, micro=10^-6, milli=10^-3, centi=10^-2, kilo=10^3, mega=10^6, giga=10^9, tera=10^12
+
+**Given** an SI prefix applied to a unit where it does not make sense (e.g., `kilofahrenheit`)
+**When** the engine encounters it
+**Then** it does not recognize the combination as a valid unit
+**And** it returns an appropriate error or treats the input as text