1. **State the problem:** We need to find the circumference of a circle given its diameter is 30 mm. 2. **Formula:** The circumference $C$ of a circle is given by the formula: $$C = \pi d$$ where $d$ is the diameter. 3. **Given:** $d = 30$ mm and $\pi = 3.14$. 4. **Calculate the circumference:** $$C = 3.14 \times 30$$ 5. **Perform the multiplication:** $$C = 94.2$$ 6. **Include the unit:** Since diameter is in millimeters, circumference is also in millimeters. **Final answer:** $$\boxed{94.2\ \text{mm}}$$