diff --git a/.gitignore b/.gitignore
index d66fcf8..9fb1b1b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 /bin
 /releases
+/vendor
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index bce8fc2..f4427e8 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -20,7 +20,7 @@ Please note that this project is released with a [Contributor Code of Conduct][c
 
 Here are a few things you can do that will increase the likelihood of your pull request being accepted:
 
-- Make sure that your code is formatted correctly: `make gofmt`.
+- Make sure that your code is formatted correctly according to `go fmt`: `go fmt .`.
 - Write tests.
 - Keep your change as focused as possible. If there are multiple changes you would like to make that are not dependent upon each other, consider submitting them as separate pull requests.
 - Write a [good commit message](http://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
diff --git a/Makefile b/Makefile
index cf910f1..ff80eaf 100644
--- a/Makefile
+++ b/Makefile
@@ -3,30 +3,21 @@ GO111MODULES := 1
 export GO111MODULES
 
 GO := $(CURDIR)/script/go
-GOFMT := $(CURDIR)/script/gofmt
 
 GO_LDFLAGS := -X main.BuildVersion=$(shell git describe --tags --always --dirty || echo unknown)
-GOFLAGS := -ldflags "$(GO_LDFLAGS)"
+GOFLAGS := -mod=readonly -ldflags "$(GO_LDFLAGS)"
 
 ifdef USE_ISATTY
 GOFLAGS := $(GOFLAGS) --tags isatty
 endif
 
-GO_SRCS := $(sort $(shell $(GO) list -f ' \
-	{{$$ip := .Dir}} \
-	{{range .GoFiles     }}{{printf "%s/%s\n" $$ip .}}{{end}} \
-	{{range .CgoFiles    }}{{printf "%s/%s\n" $$ip .}}{{end}} \
-	{{range .TestGoFiles }}{{printf "%s/%s\n" $$ip .}}{{end}} \
-	{{range .XTestGoFiles}}{{printf "%s/%s\n" $$ip .}}{{end}} \
-	' ./...))
-
 .PHONY: all
 all: bin/git-sizer
 
 .PHONY: bin/git-sizer
 bin/git-sizer:
 	mkdir -p bin
-	$(GO) build $(GOFLAGS) -o $@ $(PACKAGE)
+	$(GO) build $(GOFLAGS) -o $@ .
 
 # Cross-compile for a bunch of common platforms. Note that this
 # doesn't work with USE_ISATTY:
@@ -50,7 +41,7 @@ define PLATFORM_template =
 .PHONY: bin/git-sizer-$(1)-$(2)$(3)
 bin/git-sizer-$(1)-$(2)$(3):
 	mkdir -p bin
-	GOOS=$(1) GOARCH=$(2) $$(GO) build $$(GOFLAGS) -ldflags "-X main.ReleaseVersion=$$(VERSION)" -o $$@ $$(PACKAGE)
+	GOOS=$(1) GOARCH=$(2) $$(GO) build $$(GOFLAGS) -ldflags "-X main.ReleaseVersion=$$(VERSION)" -o $$@ .
 common-platforms: bin/git-sizer-$(1)-$(2)$(3)
 
 # Note that releases don't include code from vendor (they're only used
@@ -72,8 +63,8 @@ endef
 $(eval $(call PLATFORM_template,linux,amd64))
 $(eval $(call PLATFORM_template,linux,386))
 
-$(eval $(call PLATFORM_template,darwin,386))
 $(eval $(call PLATFORM_template,darwin,amd64))
+$(eval $(call PLATFORM_template,darwin,arm64))
 
 $(eval $(call PLATFORM_template,windows,amd64,.exe))
 $(eval $(call PLATFORM_template,windows,386,.exe))
@@ -85,23 +76,6 @@ test: bin/git-sizer gotest
 gotest:
 	$(GO) test -timeout 60s $(GOFLAGS) ./...
 
-.PHONY: gofmt
-gofmt:
-	$(GOFMT) -l -w $(GO_SRCS) | sed -e 's/^/Fixing /'
-
-.PHONY: goimports
-goimports:
-	goimports -l -w -e $(GO_SRCS)
-
-.PHONY: govet
-govet:
-	$(GO) vet ./...
-
 .PHONY: clean
 clean:
 	rm -rf bin
-
-# List all of this project's Go sources:
-.PHONY: srcs
-srcs:
-	@printf "%s\n" $(GO_SRCS)
diff --git a/docs/BUILDING.md b/docs/BUILDING.md
index d215c80..a977a2c 100644
--- a/docs/BUILDING.md
+++ b/docs/BUILDING.md
@@ -25,7 +25,7 @@ This procedure is intended for experts and people who want to help develop `git-
         git clone https://github.com/github/git-sizer.git
         cd git-sizer
 
-2.  Install Go if necessary and create and prepare a project-local `GOPATH`:
+2.  Install Go if necessary:
 
         script/bootstrap
 
diff --git a/go.mod b/go.mod
index 20bf4b8..59bdcf6 100644
--- a/go.mod
+++ b/go.mod
@@ -1,6 +1,6 @@
 module github.com/github/git-sizer
 
-go 1.13
+go 1.16
 
 require (
 	github.com/davecgh/go-spew v1.1.1 // indirect
diff --git a/script/ensure-go-installed.sh b/script/ensure-go-installed.sh
index 43ba12d..3111b9e 100644
--- a/script/ensure-go-installed.sh
+++ b/script/ensure-go-installed.sh
@@ -4,17 +4,17 @@ if [ -z "$ROOTDIR" ]; then
     echo 1>&2 'ensure-go-installed.sh invoked without ROOTDIR set!'
 fi
 
-# Is go installed, and at least 1.13?
+# Is go installed, and at least 1.16?
 go_ok() {
     set -- $(go version 2>/dev/null |
                 sed -n 's/.*go\([0-9][0-9]*\)\.\([0-9][0-9]*\).*/\1 \2/p' |
                 head -n 1)
-    [ $# -eq 2 ] && [ "$1" -eq 1 ] && [ "$2" -ge 13 ]
+    [ $# -eq 2 ] && [ "$1" -eq 1 ] && [ "$2" -ge 16 ]
 }
 
 # If a local go is installed, use it.
 set_up_vendored_go() {
-    GO_VERSION=go1.13.4
+    GO_VERSION=go1.16.3
     VENDORED_GOROOT="$ROOTDIR/vendor/$GO_VERSION/go"
     if [ -x "$VENDORED_GOROOT/bin/go" ]; then
         export GOROOT="$VENDORED_GOROOT"
diff --git a/script/install-vendored-go b/script/install-vendored-go
index 34bdf88..2407618 100755
--- a/script/install-vendored-go
+++ b/script/install-vendored-go
@@ -1,19 +1,27 @@
 #!/bin/sh
 
 # The checksums below must correspond to the downloads for this version.
-GO_VERSION=go1.13.4
+GO_VERSION=go1.16.3
 
-if [ $(uname -s) = "Darwin" ]; then
-    GO_PKG=${GO_VERSION}.darwin-amd64.tar.gz
-    GO_PKG_SHA=9f0721551a24a1eb43d2005cd58bd7b17574e50384b8da8896b0754259790752
-elif [ $(uname -s) = "Linux" ]; then
+case "$(uname -s):$(uname -m)" in
+Linux:x86_64)
     GO_PKG=${GO_VERSION}.linux-amd64.tar.gz
-    GO_PKG_SHA=692d17071736f74be04a72a06dab9cac1cd759377bd85316e52b2227604c004c
-else
+    GO_PKG_SHA=951a3c7c6ce4e56ad883f97d9db74d3d6d80d5fec77455c6ada6c1f7ac4776d2
+    ;;
+Darwin:x86_64)
+    GO_PKG=${GO_VERSION}.darwin-amd64.tar.gz
+    GO_PKG_SHA=6bb1cf421f8abc2a9a4e39140b7397cdae6aca3e8d36dcff39a1a77f4f1170ac
+    ;;
+Darwin:arm64)
+    GO_PKG=${GO_VERSION}.darwin-arm64.tar.gz
+    GO_PKG_SHA=f4e96bbcd5d2d1942f5b55d9e4ab19564da4fad192012f6d7b0b9b055ba4208f
+    ;;
+*)
     echo 1>&2 "I don't know how to install Go on your platform."
     echo 1>&2 "Please install $GO_VERSION or later and add it to your PATH."
     exit 1
-fi
+    ;;
+esac
 
 archivesum() {
     shasum -a256 "$ARCHIVE"