ParticleSystem.Particle.startLifetime

Description

The starting lifetime of the particle.

This is the total lifetime of this particle in seconds. The Particle System sets this value when it first spawns the particle.

using UnityEngine;

public class Example : MonoBehaviour
{
    void Start()
    {
        ParticleSystem.Particle particle = new ParticleSystem.Particle();

        // Calculate how long the particle has been alive.
        float timeAlive = particle.startLifetime - particle.lifetime;
    }
}