diff --git a/internal/utils/varint.go b/internal/utils/varint.go
new file mode 100644
index 00000000..35e8674e
--- /dev/null
+++ b/internal/utils/varint.go
@@ -0,0 +1,101 @@
+package utils
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+
+	"github.com/lucas-clemente/quic-go/internal/protocol"
+)
+
+// taken from the QUIC draft
+const (
+	maxVarInt1 = 63
+	maxVarInt2 = 16383
+	maxVarInt4 = 1073741823
+	maxVarInt8 = 4611686018427387903
+)
+
+// ReadVarInt reads a number in the QUIC varint format
+func ReadVarInt(b io.ByteReader) (uint64, error) {
+	firstByte, err := b.ReadByte()
+	if err != nil {
+		return 0, err
+	}
+	// the first two bits of the first byte encode the length
+	len := 1 << ((firstByte & 0xc0) >> 6)
+	b1 := firstByte & (0xff - 0xc0)
+	if len == 1 {
+		return uint64(b1), nil
+	}
+	b2, err := b.ReadByte()
+	if err != nil {
+		return 0, err
+	}
+	if len == 2 {
+		return uint64(b2) + uint64(b1)<<8, nil
+	}
+	b3, err := b.ReadByte()
+	if err != nil {
+		return 0, err
+	}
+	b4, err := b.ReadByte()
+	if err != nil {
+		return 0, err
+	}
+	if len == 4 {
+		return uint64(b4) + uint64(b3)<<8 + uint64(b2)<<16 + uint64(b1)<<24, nil
+	}
+	b5, err := b.ReadByte()
+	if err != nil {
+		return 0, err
+	}
+	b6, err := b.ReadByte()
+	if err != nil {
+		return 0, err
+	}
+	b7, err := b.ReadByte()
+	if err != nil {
+		return 0, err
+	}
+	b8, err := b.ReadByte()
+	if err != nil {
+		return 0, err
+	}
+	return uint64(b8) + uint64(b7)<<8 + uint64(b6)<<16 + uint64(b5)<<24 + uint64(b4)<<32 + uint64(b3)<<40 + uint64(b2)<<48 + uint64(b1)<<56, nil
+}
+
+// WriteVarInt writes a number in the QUIC varint format
+func WriteVarInt(b *bytes.Buffer, i uint64) {
+	if i <= maxVarInt1 {
+		b.WriteByte(uint8(i))
+	} else if i <= maxVarInt2 {
+		b.Write([]byte{uint8(i>>8) | 0x40, uint8(i)})
+	} else if i <= maxVarInt4 {
+		b.Write([]byte{uint8(i>>24) | 0x80, uint8(i >> 16), uint8(i >> 8), uint8(i)})
+	} else if i <= maxVarInt8 {
+		b.Write([]byte{
+			uint8(i>>56) | 0xc0, uint8(i >> 48), uint8(i >> 40), uint8(i >> 32),
+			uint8(i >> 24), uint8(i >> 16), uint8(i >> 8), uint8(i),
+		})
+	} else {
+		panic(fmt.Sprintf("%#x doesn't fit into 62 bits", i))
+	}
+}
+
+// VarIntLen determines the number of bytes that will be needed to write a number
+func VarIntLen(i uint64) protocol.ByteCount {
+	if i <= maxVarInt1 {
+		return 1
+	}
+	if i <= maxVarInt2 {
+		return 2
+	}
+	if i <= maxVarInt4 {
+		return 4
+	}
+	if i <= maxVarInt8 {
+		return 8
+	}
+	panic(fmt.Sprintf("%#x doesn't fit into 62 bits", i))
+}
diff --git a/internal/utils/varint_test.go b/internal/utils/varint_test.go
new file mode 100644
index 00000000..b49774f0
--- /dev/null
+++ b/internal/utils/varint_test.go
@@ -0,0 +1,157 @@
+package utils
+
+import (
+	"bytes"
+
+	. "github.com/onsi/ginkgo"
+	. "github.com/onsi/gomega"
+)
+
+var _ = Describe("Varint encoding / decoding", func() {
+	Context("decoding", func() {
+		It("reads a 1 byte number", func() {
+			b := bytes.NewReader([]byte{25}) // 00011001
+			val, err := ReadVarInt(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(val).To(Equal(uint64(25)))
+			Expect(b.Len()).To(BeZero())
+		})
+
+		It("reads a number that is encoded too long", func() {
+			b := bytes.NewReader([]byte{0x40, 0x25}) // first byte: 01000000
+			val, err := ReadVarInt(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(val).To(Equal(uint64(37)))
+			Expect(b.Len()).To(BeZero())
+		})
+
+		It("reads a 2 byte number", func() {
+			b := bytes.NewReader([]byte{0x7b, 0xbd}) // first byte: 01111011
+			val, err := ReadVarInt(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(val).To(Equal(uint64(15293)))
+			Expect(b.Len()).To(BeZero())
+		})
+
+		It("reads a 4 byte number", func() {
+			b := bytes.NewReader([]byte{0x9d, 0x7f, 0x3e, 0x7d}) // first byte: 10011011
+			val, err := ReadVarInt(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(val).To(Equal(uint64(494878333)))
+			Expect(b.Len()).To(BeZero())
+		})
+
+		It("reads an 8 byte number", func() {
+			b := bytes.NewReader([]byte{0xc2, 0x19, 0x7c, 0x5e, 0xff, 0x14, 0xe8, 0x8c}) // first byte: 10000010
+			val, err := ReadVarInt(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(val).To(Equal(uint64(151288809941952652)))
+			Expect(b.Len()).To(BeZero())
+		})
+	})
+
+	Context("encoding", func() {
+		It("writes a 1 byte number", func() {
+			b := &bytes.Buffer{}
+			WriteVarInt(b, 37)
+			Expect(b.Bytes()).To(Equal([]byte{0x25}))
+		})
+
+		It("writes the maximum 1 byte number in 1 byte", func() {
+			b := &bytes.Buffer{}
+			WriteVarInt(b, maxVarInt1)
+			Expect(b.Bytes()).To(Equal([]byte{0x3f /* 00111111 */}))
+		})
+
+		It("writes the minimum 2 byte number in 2 bytes", func() {
+			b := &bytes.Buffer{}
+			WriteVarInt(b, maxVarInt1+1)
+			Expect(b.Bytes()).To(Equal([]byte{0x40, maxVarInt1 + 1}))
+		})
+
+		It("writes a 2 byte number", func() {
+			b := &bytes.Buffer{}
+			WriteVarInt(b, 15293)
+			Expect(b.Bytes()).To(Equal([]byte{0x7b, 0xbd}))
+		})
+
+		It("writes the maximum 2 byte number in 2 bytes", func() {
+			b := &bytes.Buffer{}
+			WriteVarInt(b, maxVarInt2)
+			Expect(b.Bytes()).To(Equal([]byte{0x7f /* 01111111 */, 0xff}))
+		})
+
+		It("writes the minimum 4 byte number in 4 bytes", func() {
+			b := &bytes.Buffer{}
+			WriteVarInt(b, maxVarInt2+1)
+			Expect(b.Len()).To(Equal(4))
+			num, err := ReadVarInt(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(num).To(Equal(uint64(maxVarInt2 + 1)))
+		})
+
+		It("writes a 4 byte number", func() {
+			b := &bytes.Buffer{}
+			WriteVarInt(b, 494878333)
+			Expect(b.Bytes()).To(Equal([]byte{0x9d, 0x7f, 0x3e, 0x7d}))
+		})
+
+		It("writes the maximum 4 byte number in 4 bytes", func() {
+			b := &bytes.Buffer{}
+			WriteVarInt(b, maxVarInt4)
+			Expect(b.Bytes()).To(Equal([]byte{0xbf /* 10111111 */, 0xff, 0xff, 0xff}))
+		})
+
+		It("writes the minimum 8 byte number in 8 bytes", func() {
+			b := &bytes.Buffer{}
+			WriteVarInt(b, maxVarInt4+1)
+			Expect(b.Len()).To(Equal(8))
+			num, err := ReadVarInt(b)
+			Expect(err).ToNot(HaveOccurred())
+			Expect(num).To(Equal(uint64(maxVarInt4 + 1)))
+		})
+
+		It("writes an 8 byte number", func() {
+			b := &bytes.Buffer{}
+			WriteVarInt(b, 151288809941952652)
+			Expect(b.Bytes()).To(Equal([]byte{0xc2, 0x19, 0x7c, 0x5e, 0xff, 0x14, 0xe8, 0x8c}))
+		})
+
+		It("writes the maximum 8 byte number in 8 bytes", func() {
+			b := &bytes.Buffer{}
+			WriteVarInt(b, maxVarInt8)
+			Expect(b.Bytes()).To(Equal([]byte{0xff /* 11111111 */, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff}))
+		})
+
+		It("panics when given a too large number (> 62 bit)", func() {
+			b := &bytes.Buffer{}
+			Expect(func() { WriteVarInt(b, maxVarInt8+1) }).Should(Panic())
+		})
+	})
+
+	Context("determining the length needed for encoding", func() {
+		It("for numbers that need 1 byte", func() {
+			Expect(VarIntLen(0)).To(BeEquivalentTo(1))
+			Expect(VarIntLen(maxVarInt1)).To(BeEquivalentTo(1))
+		})
+
+		It("for numbers that need 2 bytes", func() {
+			Expect(VarIntLen(maxVarInt1 + 1)).To(BeEquivalentTo(2))
+			Expect(VarIntLen(maxVarInt2)).To(BeEquivalentTo(2))
+		})
+
+		It("for numbers that need 4 bytes", func() {
+			Expect(VarIntLen(maxVarInt2 + 1)).To(BeEquivalentTo(4))
+			Expect(VarIntLen(maxVarInt4)).To(BeEquivalentTo(4))
+		})
+
+		It("for numbers that need 8 bytes", func() {
+			Expect(VarIntLen(maxVarInt4 + 1)).To(BeEquivalentTo(8))
+			Expect(VarIntLen(maxVarInt8)).To(BeEquivalentTo(8))
+		})
+
+		It("panics when given a too large number (> 62 bit)", func() {
+			Expect(func() { VarIntLen(maxVarInt8 + 1) }).Should(Panic())
+		})
+	})
+})