Ampere is a unit of measurement used in electricity to express the intensity of an electric current. Current is a direct function of the amount of electrons passing through a powered conductor and closing on a load. Knowing this information is very useful in particular when connecting a device to the power supply network, where we will be dealing with a current alternative passing power from a supplier to a home appliance.
Steps
Method 1 of 3: Convert Watts to Amps
Step 1. Apply the conversion formula for direct current
The electric current, represented by the physical quantity I, is measured in Amps (TO), and can be obtained by dividing the power P consumed by a load and expressed in Watts (W) by tension U which supplies it, expressed in Volts (V). We represent this by the following formula:

I_{(TO)} = P_{(W)} / V_{(V)}
or, more simply: current (A) = power (W) / voltage (V).
Step 2. Get an idea of what the power factor is
The power factor or Cosine φ (PHI), is a phenomenon well known to industrial electricians working on electric motors in alternating current mode. It is physically due to the phase angle between voltage and current in an inductive load, such as a refrigerator, washing machine, or fan, and is often also a factor in raising electricity bills if it is not properly compensated. It represents the ratio between the power real Where active necessary to perform a job (hence the term Active power), which is noted P and the apparent power, noted S which is that supplied to a circuit operating in an alternating current regime and is expressed by a coefficient varying between 0 and 1. Note that a power factor can be considered as decent when it is between 0, 8 and 1. The power factor, traditionally noted λ Where Cos φ is calculated by dividing the active power (P) expressed in Watts by the apparent power (S) measured in VoltAmps Where GO that is:
λ = P / S
Step 3. Determine a power factor by calculating the apparent power
Apparent power can be determined by S = V _{eff} x I _{eff}
where S represents the apparent power in VoltAmperes (VA), V _{eff} the value of the rms supply voltage and I _{eff} the value of the rms current supplied by the source. These last two parameters can be resolved as follows:
 V _{eff} = V _{Crete} / √2 in volts (V);
 I _{eff} = I _{Crete} / √2 in amperes (A).
Step 4. Use the single phase AC power factor
The singlephase current, represented by the symbol I is measured in Amps (A) and can be calculated by dividing the value of the actual power (P) measured in Watts (W) by the power factor (λ) multiplied by the value of the effective voltage (V_{eff}) measured in Volts (V). We illustrate this as follows:

I_{(TO)} = P_{(W)} / (λ x V_{(V)}
or more simply: I (_{TO}) = P (_{W}) / λ x V (_{V}).
Method 2 of 3: Measure DC current with a multimeter
Step 1. Verify that the circuit to be tested is operating in direct current
Note beforehand that we have used the symbol so far V to represent the voltages in alternating mode. In continuous mode, we will traditionally use the symbol U to represent them. Direct current, as the name suggests, is not subject to any cyclic variation and only flows in one direction. If the circuit you want to work on is battery powered, it will run continuously.
In most countries, electricity is supplied to homes or businesses in the form of alternating currents. The alternating current can be easily converted into low voltage direct current by means of a transformer and a rectifier bridge followed by a filtering system
Step 2. Determine the path traveled by the current
To be able to measure the intensity of the current flowing through a circuit, you will need to insert an ammeter into it. Follow the wires coming out of the positive and negative poles of the cell or battery you are using to power your circuit to locate the path through which the current to be measured passes.
Step 3. Verify that your circuit is working
If the circuit is broken or the battery is dead, your ammeter will not be able to measure the current flowing through it or will measure it inefficiently. Power up your circuit to verify that it is functioning normally.
Step 4. Turn off your circuit
For the most basic circuits, this can mean a complete removal of its battery. With more powerful cells or batteries, there may be a risk of electric shock to you. Take care to check that the circuit has been deenergized and if you are not sure, wear a pair of insulating gloves to avoid any risk of electric shock.
Step 5. Connect the positive terminal of your ammeter
Please note before any manipulation that your multimeter must be positioned in mode Ammeter on its highest caliber and that under these conditions, you should not NEVER connect it directly between the terminals of the voltage source, otherwise you will shortcircuit it and put your device out of service. Your measuring device will have been delivered to you with two cords ending with test probes: one red and the other black. The red lead corresponds to the positive pole (+) of the device and the black one to its negative () terminal. Disconnect the circuit from the positive terminal of your battery to which you will connect the red cable of your multimeter.
 Ammeter must be inserted serial in the circuit whose consumption you want to measure. This means that the current coming from the battery will enter through the positive pole of the ammeter and exit through its negative terminal to the circuit to be powered. When you have closed and powered your circuit, you will be able to see the value of the measured current displayed on the screen of the device.
Step 6. Now close the circuit with the black cable from your multimeter
Now connect the black cable from your ammeter to the wire from your circuit that previously went to the positive terminal of the battery in order to close the circuit and allow current to flow through it. Your measuring device is now an integral part of the circuit supply loop.
Step 7. Power up your assembly
This means you need to reinstall the battery and your circuit should start working again. You should see a value in Amperes displayed on your ammeter screen (TO) or milliamps (my) for circuits that consume little current.
Method 3 of 3: Apply Ohm's law to calculate a current
Step 1. Familiarize yourself with the concept of Ohm's Law
Ohm's law is THE fundamental principle that applies to any application based on electricity or electronics. It establishes a relationship between the current flowing through a conductive element and the voltage supplying the circuit thus formed. Ohm's law is represented by the formula U = R x I, from which derive mathematically R = U / I and I = U / R. The symbols used in these equations are defined as follows:
 U is the voltage expressed in Volts between two points of a circuit;
 R is the resistance expressed in Ohms Where Ω applied between these two points;
 I is the current intensity expressed in Amps Where TO crossing resistance.
Step 2. Determine the voltage across the assembly
If the latter works with a 9 volt battery, part of the equation is already solved. It is easy to know the nominal voltage delivered by a battery that you are using by examining the packaging in which it was delivered to you or by doing a quick consultation on the Internet. With a little more experience, you will be able to measure your battery voltage with your multimeter.
Common cylindrical cells (from size AAA to D) deliver a nominal voltage when new of approximately 1.5 volts
Step 3. Determine the resistors making up your circuit
You will need to know what types of components your circuit is made of and what resistance they offer to the flow of current through it. All assemblies are not designed in the same way, and some of them are not composed of resistors, you will have to learn about the circuit you want to test, locate the resistors that compose it and know their values in Ohms (Ω).
 The wiring through which the current must pass will also present a resistance which should be negligible, unless the circuit is really badly designed, damaged, or has to carry electric current very long distances.
 The formula for calculating the [resistivity of a conductor] is expressed as follows: resistance = (resistivity x length of the conductor) / area of the cross section of the conductor. Resistivity is traditionally noted ρ and the formula used will therefore be R = (ρ x L) / S, where R is the resistance in Ohms, ρ the resistivity of the material used expressed in Ohms per meter, L its length in meters and S the area of the section of the cable used expressed in m^{2}.
Step 4. Apply Ohm's Law
The voltage of a battery being applied to the circuit that it must supply, you will have to estimate the current which crosses it by dividing its voltage by the sum of the values of each resistance whose value is measured in Ohms. The result of this operation will give you the current (I) expressed in Amps (TO), which can also be solved, if several resistors are connected in series, as follows:
 (U / R_{1}) + (U / R_{2}) + (U / R_{3}), or U represents the total voltage and R the value of each resistor expressed in Ohms.
Warnings
 You must take all necessary precautions if you handle electrical circuits. If you are required to work at high voltages, you could be subjected to an electric shock or cause a fire to start. It is strongly recommended to wear insulating gloves.