diff --git a/lib/progress.ts b/lib/progress.ts
index 061a2ca7874ff8f0be9f38d1224ee9e5f014945f..f43bae9900a26396d4f888dd659733032f8bad0b 100644
--- a/lib/progress.ts
+++ b/lib/progress.ts
@@ -453,7 +453,7 @@ function newNode<Result, EventMap extends Events.Map>(
       estimator = estimateCompletion;
     } else estimator = new Ema();
   }
-  estimator?.sample(Date.now(), 0);
+  estimator?.sample(Date.now(), 0, 0);
 
   function mutate(key: keyof ReadOnlyNode) {
     if (state.detached) return;
@@ -464,7 +464,7 @@ function newNode<Result, EventMap extends Events.Map>(
       // originally, auto-estimation would use the Root's timing primitives,
       // but this makes it so that `state.estimatedTime` is in terms of the
       // Root, which is unintuitive for everyone but the renderer.
-      const time = estimator.sample(Date.now(), state.value / state.total);
+      const time = estimator.sample(Date.now(), state.value, state.total);
       if (time && (!state.estimatedTime || state.estimatedTime !== time)) {
         state.estimatedTime = time;
         owner.emit("node-change", state, "estimatedTime");
@@ -1671,9 +1671,10 @@ class KeyPool<T extends number> {
 export interface EstimationAlgorithm {
   /**
    * given a timestamp `time` and a progress value `progress`, return the
-   * timestamp that the action will be finished on.
+   * timestamp that the action will be finished on. called with (t, 0, 0)
+   * to initialize the algorithm.
    */
-  sample(time: number, progress: number): number | null;
+  sample(time: number, current: number, total: number): number | null;
 }
 
 /**
@@ -1685,33 +1686,29 @@ export class Ema implements EstimationAlgorithm {
   start: number | null = null;
   estimate: number | null = null;
   /** smoothing factor (0-1) */
-  alpha: number = 0.25;
+  alpha: number;
 
-  constructor(alpha: number = 0.1) {
+  constructor(alpha: number = 0.25) {
     this.alpha = alpha;
   }
 
-  /**
-   * given a timestamp `time` and a progress value `progress`, return the
-   * timestamp that the action will be finished on.
-   */
-  sample(time: number, progress: number): number | null {
-    ASSERT(
-      progress >= 0 && progress < 1,
-      `progress must be a number between 0 and 1, got ${progress}`,
-    );
-    if (this.start == null) {
+  sample(time: number, current: number, total: number): number | null {
+    ASSERT(current >= 0 && current <= total);
+    if (this.start == null || current === 0) {
       this.start = time;
+      this.estimate = null;
       return null;
     }
 
-    const value = (time - this.start) / progress;
+    const elapsed = time - this.start;
+    if (elapsed === 0) return null; // too quick
+
+    const rate = current / elapsed;
     this.estimate = this.estimate != null
-      ? ((1 - this.alpha) * this.estimate +
-        this.alpha * value)
-      : value;
-
-    return time + (1 - progress) * this.estimate;
+      ? (1 - this.alpha) * this.estimate + this.alpha * rate
+      : rate;
+    const remaining = total - current;
+    return time + remaining / this.estimate;
   }
 
   reset(): void {