{ "id": "amp", "version": "eb3b3813a0e7", "status": "published", "name": "Amp Calculator", "question": "How many amps does it draw?", "summary": "Works out the current in amps from watts and volts (DC or single-phase AC with a power factor), from volts and ohms, or from watts and ohms, with the other circuit values.", "category": "physics", "subcategory": "electricity", "url": "https://www.acalculator.org/physics/amp-calculator", "markdown": "https://www.acalculator.org/physics/amp-calculator.md", "kind": "function", "method": "Watts and volts: I = P ÷ V (DC), I = P ÷ (V × pf) (single-phase AC). Volts and ohms: I = V ÷ R, P = V² ÷ R. Watts and ohms: I = √(P ÷ R), V = √(P × R).", "assumptions": [ "AC values are RMS (the usual outlet and nameplate values) on a single-phase supply; three-phase circuits are not covered.", "The resistance modes treat the load as a pure resistance (power factor 1)." ], "inputs": { "$schema": "https://json-schema.org/draft/2020-12/schema", "type": "object", "properties": { "from": { "title": "Work out amps from", "description": "Which two values you know.", "type": "string", "enum": [ "pv", "vr", "pr" ] }, "cur": { "title": "Current", "description": "Direct current (batteries, USB, solar) or single-phase alternating current (wall outlets). It matters only for watts and volts.", "type": "string", "enum": [ "dc", "ac" ] }, "pf": { "title": "Power factor", "description": "The share of the volt-amperes that does work: 1 for heaters and bulbs, about 0.8 to 0.95 for motors.", "type": "number", "minimum": 0.01, "maximum": 1 }, "p": { "title": "Power", "description": "The real power the load uses.", "type": [ "number", "string" ], "x-quantity": "power", "minimum": 0.000001, "maximum": 1000000 }, "v": { "title": "Voltage", "description": "The voltage across the load (RMS volts for AC).", "type": [ "number", "string" ], "x-quantity": "voltage", "minimum": 0.000001, "maximum": 1000000 }, "r": { "title": "Resistance", "description": "The resistance of the load.", "type": [ "number", "string" ], "x-quantity": "resistance", "minimum": 0.000001, "maximum": 1000000 } } }, "outputs": { "amps": { "label": "Current", "description": "The current through the load, in amperes.", "format": "number" }, "milliamps": { "label": "Current (mA)", "description": "The current × 1,000.", "format": "number" }, "watts": { "label": "Power", "description": "The real power: V × I (× power factor for AC).", "format": "number" }, "volts": { "label": "Voltage", "description": "The voltage across the load.", "format": "number" }, "ohms": { "label": "Resistance", "description": "V ÷ I for a DC or resistive load.", "format": "number" }, "va": { "label": "Apparent power", "description": "Volts × amps, in volt-amperes.", "format": "number" } }, "defaultAnswer": { "inputs": { "from": "pv", "cur": "ac", "pf": 1, "p": 1500, "v": 120, "r": 10 }, "outputs": { "amps": 12.5, "milliamps": 12500, "watts": 1500, "volts": 120, "ohms": 9.6, "va": 1500 }, "text": "The load draws 12.5 A at 120 V and 1,500 W." }, "examples": [ { "given": { "from": "pv", "cur": "ac", "pf": 1, "p": 1500, "v": 120 }, "expect": { "amps": 12.5, "r": 9.6 }, "source": "OpenStax, University Physics Volume 2, §9.5 Electrical Energy and Power (P = IV = I²R = V²/R), https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power; hand calculation in content.mdx: 1,500 ÷ 120 = 12.5 A; 120 ÷ 12.5 = 9.6 Ω", "tolerance": 1e-12 }, { "given": { "from": "pv", "cur": "dc", "p": 2300, "v": 115 }, "expect": { "amps": 20 }, "source": "OpenStax, University Physics Volume 2, §9.5 Electrical Energy and Power (P = IV = I²R = V²/R), https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power (Example 9.9: 20.00 A at 115 V is 2,300 W); hand calculation in content.mdx: 2,300 ÷ 115 = 20 A", "tolerance": 1e-12 }, { "given": { "from": "pv", "cur": "ac", "pf": 0.8, "p": 960, "v": 120 }, "expect": { "amps": 10, "va": 1200 }, "source": "OpenStax, University Physics Volume 2, §15.4 Power in an AC Circuit (average power = I_rms V_rms cos φ), https://openstax.org/books/university-physics-volume-2/pages/15-4-power-in-an-ac-circuit; hand calculation in content.mdx: 960 ÷ (120 × 0.8) = 10 A", "tolerance": 1e-12 }, { "given": { "from": "vr", "cur": "dc", "v": 12, "r": 4 }, "expect": { "amps": 3, "p": 36 }, "source": "OpenStax, University Physics Volume 2, §9.4 Ohm’s Law (V = IR), https://openstax.org/books/university-physics-volume-2/pages/9-4-ohms-law; hand calculation in content.mdx: 12 ÷ 4 = 3 A; 12² ÷ 4 = 36 W" }, { "given": { "from": "pr", "cur": "dc", "p": 100, "r": 25 }, "expect": { "amps": 2, "v": 50 }, "source": "OpenStax, University Physics Volume 2, §9.5 Electrical Energy and Power (P = IV = I²R = V²/R), https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power (P = I²R); hand calculation in content.mdx: √(100 ÷ 25) = 2 A; √(100 × 25) = 50 V", "tolerance": 1e-12 } ], "sources": [ "OpenStax, University Physics Volume 2, §9.4 Ohm’s Law (V = IR). https://openstax.org/books/university-physics-volume-2/pages/9-4-ohms-law (retrieved 2026-10-01)", "OpenStax, University Physics Volume 2, §9.5 Electrical Energy and Power (P = IV = I²R = V²/R; Example 9.9). https://openstax.org/books/university-physics-volume-2/pages/9-5-electrical-energy-and-power (retrieved 2026-10-01)", "OpenStax, University Physics Volume 2, §15.4 Power in an AC Circuit (average power = I_rms V_rms cos φ, where cos φ is the power factor). https://openstax.org/books/university-physics-volume-2/pages/15-4-power-in-an-ac-circuit (retrieved 2026-10-01)" ], "related": [ "ohms-law", "power", "voltage-drop" ], "changelog": [] }