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Collider

Type	Class (Component, base class)
Namespace	`UnityEngine`
Inherits	Component > Object
Subclasses	BoxCollider, SphereCollider, CapsuleCollider, MeshCollider, TerrainCollider, CharacterController
2D version	`Collider2D`

Collider

A Collider gives an object an invisible physical shape. It's what raycasts hit, what Rigidbodies bump into, and what triggers detect. The shape doesn't have to match the visible mesh exactly; simple shapes are faster.

Collider types

Type	Shape	Speed	Use for
BoxCollider	Box	Fast	Crates, walls, buildings
SphereCollider	Ball	Fastest	Balls, pickup ranges
CapsuleCollider	Pill	Fast	Characters
MeshCollider	Matches a mesh	Slow	Complex static level geometry. Needs Convex on for dynamic Rigidbodies.

For complex moving objects, add several primitive colliders on child objects (a compound collider) instead of one Mesh Collider.

Properties

Name	Type	Description
`enabled`	`bool`	Turn the collider on/off.
`isTrigger`	`bool`	If true, nothing bounces off it; it only reports when things enter/exit.
`bounds`	`Bounds`	The world-space box that fully contains the collider.
`attachedRigidbody`	`Rigidbody`	The Rigidbody it belongs to (can be on a parent), or `null`.
`sharedMaterial`	`PhysicsMaterial`	Friction and bounciness. (Called `PhysicMaterial` before Unity 6.)

Methods

Method	Returns	Description
`ClosestPoint(Vector3 position)`	`Vector3`	The point on (or in) the collider closest to a position.
`Raycast(Ray ray, out RaycastHit hit, float maxDistance)`	`bool`	Raycast against only this collider.

Events (messages on your scripts)

Write these methods on a script on the same GameObject as the collider (or on the GameObject with the Rigidbody).

Message	Parameter	Called when
`OnCollisionEnter`	`Collision`	Started touching (solid).
`OnCollisionStay`	`Collision`	Every physics step while touching.
`OnCollisionExit`	`Collision`	Stopped touching.
`OnTriggerEnter`	`Collider`	Something entered a trigger.
`OnTriggerStay`	`Collider`	Every physics step while inside.
`OnTriggerExit`	`Collider`	Something left a trigger.

Useful members of Collision

Member	Description
`collision.gameObject`	The object we hit.
`collision.collider`	The exact collider we hit.
`collision.relativeVelocity`	How fast the two objects hit each other.
`collision.GetContact(0).point`	Where they touched (world space).
`collision.GetContact(0).normal`	The surface direction at that point (points away from the surface).

Code Samples

Damage zone (trigger)

using UnityEngine;

public class LavaZone : MonoBehaviour
{
    [SerializeField] private int damagePerSecond = 20;

    void OnTriggerStay(Collider other)
    {
        if (other.TryGetComponent(out Health health))
        {
            // OnTriggerStay runs each physics step, so scale by the step length.
            health.TakeDamage(damagePerSecond * Time.fixedDeltaTime);
        }
    }
}

Spawn sparks where a bullet hits

using UnityEngine;

public class ImpactEffect : MonoBehaviour
{
    [SerializeField] private GameObject sparksPrefab;

    void OnCollisionEnter(Collision collision)
    {
        ContactPoint contact = collision.GetContact(0);
        // Point the sparks out of the surface.
        Quaternion rot = Quaternion.LookRotation(contact.normal);
        Instantiate(sparksPrefab, contact.point, rot);
        Destroy(gameObject);
    }
}

Ground check with a trigger under the feet

using UnityEngine;

// Put this on a small trigger collider child object at the character's feet.
public class GroundSensor : MonoBehaviour
{
    public bool IsGrounded => contacts > 0;
    private int contacts;

    void OnTriggerEnter(Collider other) { if (!other.isTrigger) contacts++; }
    void OnTriggerExit(Collider other)  { if (!other.isTrigger) contacts--; }
}

What happens at runtime

Each physics step, the engine checks which colliders overlap or touch.
For solid collisions, it pushes Rigidbodies apart and adjusts their velocity (friction, bounce).
It compares this step's contacts with the last step's: new pairs get Enter, continuing pairs get Stay, ended pairs get Exit.
Messages are sent to scripts on both objects. On a child collider of a Rigidbody, messages go to the Rigidbody's GameObject.
Disabling or destroying a collider while inside a trigger does not call OnTriggerExit. Keep that in mind for counters like the ground sensor above.

Common Mistakes

No callback at all. Check: at least one object has a Rigidbody; both are 3D (or both 2D); the method signature is exactly right; the layers are allowed to collide in the Layer Collision Matrix.
OnTriggerEnter(Collision c): wrong parameter type. Triggers get a Collider.
Scaling colliders to negative values or very thin shapes. Causes missed collisions. Use the collider's own Size/Radius fields.

Collider

Collider types

Properties

Methods

Events (messages on your scripts)

Useful members of Collision

Code Samples

Damage zone (trigger)

Spawn sparks where a bullet hits

Ground check with a trigger under the feet

What happens at runtime

Common Mistakes

See also