diff --git a/lib/binary_parser.ts b/lib/binary_parser.ts
index cf511f6..a463181 100644
--- a/lib/binary_parser.ts
+++ b/lib/binary_parser.ts
@@ -232,6 +232,27 @@ const PRIMITIVE_SIZES: { [key in PrimitiveTypes]: number } = {
   doublebe: 8,
 };
 
+// Number of bytes consumed by consecutive bit fields, mirroring generateBit
+function bitFieldsSize(lengths: number[]): number {
+  let bytes = 0;
+  let rem = 0;
+  lengths.forEach((length, i) => {
+    if (length > rem) {
+      let maxBits = 0;
+      for (let j = i; j < lengths.length; j++) {
+        if (maxBits + lengths[j] > 32) break;
+        maxBits += lengths[j];
+      }
+      const read = Math.min(4, Math.max(1, Math.ceil((maxBits - rem) / 8)));
+      bytes += read;
+      length -= rem;
+      rem = read * 8;
+    }
+    rem -= length;
+  });
+  return bytes;
+}
+
 const PRIMITIVE_NAMES: { [key in PrimitiveTypes]: string } = {
   uint8: "Uint8",
   uint16le: "Uint16",
@@ -873,6 +894,7 @@ export class Parser {
 
   sizeOf(): number {
     let size = NaN;
+    let next = this.next;
 
     if (Object.keys(PRIMITIVE_SIZES).indexOf(this.type) >= 0) {
       size = PRIMITIVE_SIZES[this.type as PrimitiveTypes];
@@ -897,26 +919,47 @@ export class Parser {
       typeof this.options.length === "number"
     ) {
       let elementSize = NaN;
-      if (typeof this.options.type === "string") {
-        elementSize = PRIMITIVE_SIZES[this.options.type as PrimitiveTypes];
-      } else if (this.options.type instanceof Parser) {
-        elementSize = this.options.type.sizeOf();
+      const type = this.options.type;
+      if (typeof type === "string" && aliasRegistry.has(type)) {
+        elementSize = aliasRegistry.get(type)!.sizeOf();
+      } else if (typeof type === "string") {
+        elementSize = PRIMITIVE_SIZES[type as PrimitiveTypes];
+      } else if (type instanceof Parser) {
+        elementSize = type.sizeOf();
       }
       size = this.options.length * elementSize;
 
       // if this a skip
     } else if (this.type === "seek") {
-      size = this.options.length as number;
+      if (typeof this.options.length === "number") {
+        size = this.options.length;
+      }
 
       // if this is a nested parser
     } else if (this.type === "nest") {
-      size = (this.options.type as Parser).sizeOf();
-    } else if (!this.type) {
+      const type = this.options.type!;
+      size = (
+        typeof type === "string" ? aliasRegistry.get(type) : type
+      )!.sizeOf();
+
+      // if this is a run of consecutive bit fields
+    } else if (this.type === "bit") {
+      const lengths = [this.options.length as number];
+      while (next && next.type === "bit") {
+        lengths.push(next.options.length as number);
+        next = next.next;
+      }
+      size = bitFieldsSize(lengths);
+    } else if (
+      !this.type ||
+      this.type === "pointer" ||
+      this.type === "saveOffset"
+    ) {
       size = 0;
     }
 
-    if (this.next) {
-      size += this.next.sizeOf();
+    if (next) {
+      size += next.sizeOf();
     }
 
     return size;
diff --git a/test/composite_parser.ts b/test/composite_parser.ts
index b369b54..fb7f2d8 100644
--- a/test/composite_parser.ts
+++ b/test/composite_parser.ts
@@ -1170,6 +1170,49 @@ function compositeParserTests(
 
         expect(parser.sizeOf()).toStrictEqual(1 + 4 + 10 + 2 + 3 + 8);
       });
+      it("should count size for bit fields", () => {
+        const cases = [
+          [4, 4],
+          [1],
+          [3, 7],
+          [16, 8],
+          [20, 4, 8],
+          [30, 4],
+          [32, 32],
+          [7, 30, 3],
+          [12, 12, 12, 12],
+        ];
+        for (const lengths of cases) {
+          const parser: any = new Parser();
+          lengths.forEach((length, i) => parser[`bit${length}`](`b${i}`));
+          parser.uint8("x").saveOffset("end");
+          const { end } = parser.parse(new Uint8Array(32));
+          expect(parser.sizeOf()).toStrictEqual(end);
+        }
+      });
+      it("should count size for seek with non-number length as NaN", () => {
+        const parser = new Parser().uint8("x").seek("x").uint8("a");
+        expect(parser.sizeOf()).toBeNaN();
+      });
+      it("should count size for aliased nest and array types", () => {
+        new Parser().namely("sizeOfAlias").uint16("a").uint8("b");
+        expect(
+          new Parser().nest("n", { type: "sizeOfAlias" }).sizeOf(),
+        ).toStrictEqual(3);
+        expect(
+          new Parser()
+            .array("arr", { type: "sizeOfAlias", length: 2 })
+            .sizeOf(),
+        ).toStrictEqual(6);
+      });
+      it("should count pointer and saveOffset as zero bytes", () => {
+        const parser = new Parser()
+          .uint8("a")
+          .saveOffset("off")
+          .pointer("p", { type: "uint8", offset: 0 })
+          .uint16("b");
+        expect(parser.sizeOf()).toStrictEqual(3);
+      });
       it("should assert parsed values", () => {
         const parser1 = Parser.start().string("msg", {
           encoding: "utf8",