# Payload calculator

> Calculate the payload of a tool in Isaac Sim and store it in Wandelbots NOVA. Learn how to choose the reference frame, read mass, center of mass, and inertia, and load a stored payload.

The `Payload Calculator` adds up mass, center of mass, and inertia of every prim with a rigid body API in the stage.
PhysX derives the values from the colliders and the mass API attributes of each rigid body.

Open the tool on its own, for example the gripper USD file of the stacking demo.
Otherwise the robot links and the cell count as payload as well.

### Step 1: Open the payload calculator

Open `Tools` → `Wandelbots NOVA` → `Payload Calculator`.

### Step 2: Calculate the payload

1. Open the tab `Export to NOVA`.
2. In `Target`, select the `Instance` and enter the `Payload name`.
3. In `Payload`, select the `Reference Xform` and pick the point where the tool mounts on the flange.\
   → The calculator expresses the results in this frame.
4. Click `Calculate`.\
   → `Result` shows mass in kg, center of mass in mm, moments and products of inertia in kg m², and the rigid bodies used.

If a value is implausible, for example a mass of zero, a warning appears.
Review the mass API attributes and colliders of the listed rigid bodies.

![Screenshot of the Payload Calculator with the calculated payload of the Schunk gripper](/assets/nova/omniservice/payload_calculator/payload-calculator-export.webp)

### Step 3: Store the payload in Wandelbots NOVA

Click `Store Payload`.\
→ The notification `Payload '<name>' stored on <instance> in cell '<cell>'.` appears.

The payload is stored in the first cell of the instance under the key `payload/<name>`.

## Load a stored payload

1. Open the tab `Load from NOVA`.
2. Select the `Instance` and the `Payload`.
3. Click `Load Payload`.\
   → Name, mass, center of mass, and moments of inertia of the stored payload are shown.

Loading a payload only shows its values. It does not change the stage.