diff --git a/src/JsonFormatter.cpp b/src/JsonFormatter.cpp
index 3a4c20d..87b7a20 100644
--- a/src/JsonFormatter.cpp
+++ b/src/JsonFormatter.cpp
@@ -7,13 +7,14 @@ void BufferedJsonFormatter::setBuffer(char *b, size_t sz) {
   this->buff = b;
   this->buffSize = sz;
   this->buff[0] = 0;
+  this->_len = 0;
   this->_nocomma = true;
   this->_overflow = false;
   this->_objects = 0;
   this->_arrays = 0;
 }
 size_t BufferedJsonFormatter::length() const {
-  return strlen(this->buff);
+  return this->_len;
 }
 void JsonFormatter::beginObject(const char *name) {
   if(name && strlen(name) > 0) this->appendElem(name);
@@ -87,17 +88,29 @@ void JsonFormatter::addElem(const char *name, uint64_t lval) { sprintf(this->_nu
 void JsonFormatter::addElem(const char *name, bool bval) { strcpy(this->_numbuff, bval ? "true" : "false"); this->_appendNumber(name); }
 
 void JsonFormatter::_safecat(const char *val, bool escape) {
-  size_t len = (escape ? this->calcEscapedLength(val) : strlen(val)) + strlen(this->buff);
-  if(escape) len += 2;
-  if(len >= this->buffSize) {
+  const size_t vlen = escape ? this->calcEscapedLength(val) + 2 : strlen(val);
+  if(this->_len + vlen >= this->buffSize) {
     if(!this->_overflow) ESP_LOGE(TAG, "JSON exceeded buffer size %u", (unsigned)this->buffSize);
     this->_overflow = true;
     return;
   }
-  if(escape) strcat(this->buff, "\"");
-  if(escape) this->escapeString(val, &this->buff[strlen(this->buff)]);
-  else strcat(this->buff, val);
-  if(escape) strcat(this->buff, "\"");
+  this->_appendAt(val, escape, vlen);
+}
+void JsonFormatter::_appendAt(const char *val, bool escape, size_t vlen) {
+  char *p = this->buff + this->_len;
+  if(escape) {
+    *p++ = '"';
+    *p = 0;
+    this->escapeString(val, p);
+    p += vlen - 2;
+    *p++ = '"';
+  }
+  else {
+    memcpy(p, val, vlen);
+    p += vlen;
+  }
+  *p = 0;
+  this->_len += vlen;
 }
 void JsonFormatter::_appendNumber(const char *name) { this->appendElem(name); this->_safecat(this->_numbuff); } 
 size_t JsonFormatter::escapeChar(char c, char *out) {
diff --git a/src/JsonFormatter.h b/src/JsonFormatter.h
index 54f8927..3f93025 100644
--- a/src/JsonFormatter.h
+++ b/src/JsonFormatter.h
@@ -8,6 +8,8 @@ class JsonFormatter {
   protected:
     char *buff;
     size_t buffSize;
+    // What is in buff, kept so appending never has to strlen() the buffer.
+    size_t _len = 0;
     bool _headersSent = false;
     uint8_t _objects = 0;
     uint8_t _arrays = 0;
@@ -16,6 +18,9 @@ class JsonFormatter {
     bool _overflow = false;
     char _numbuff[25] = {0};
     virtual void _safecat(const char *val, bool escape = false);
+    // Appends val at buff + _len.  vlen is what it takes, including the quotes
+    // when escaped; the caller has made sure it fits.
+    void _appendAt(const char *val, bool escape, size_t vlen);
     void _appendNumber(const char *name);
   public:
     bool overflowed() const { return _overflow; }
diff --git a/src/WResp.cpp b/src/WResp.cpp
index 1d4dcde..7d00372 100644
--- a/src/WResp.cpp
+++ b/src/WResp.cpp
@@ -12,12 +12,13 @@ void JsonSockEvent::beginEvent(AsyncWebSocket *server, const char *evt, char *bu
   this->_nocomma = true;
   this->_closed = false;
   this->_overflow = false;
-  snprintf(this->buff, buffSize, "42[%s,", evt);
+  const int n = snprintf(this->buff, buffSize, "42[%s,", evt);
+  this->_len = n < 0 ? 0 : ((size_t)n < buffSize ? (size_t)n : buffSize - 1);
 }
 void JsonSockEvent::closeEvent() {
   if(!this->_closed) {
     // The ] and the terminator both have to fit.
-    if(strlen(this->buff) + 1 < buffSize) strcat(this->buff, "]");
+    if(this->_len + 1 < buffSize) this->_appendAt("]", false, 1);
     else this->_overflow = true;
   }
   this->_nocomma = true;
@@ -37,49 +38,47 @@ void JsonSockEvent::endEvent(uint32_t clientId) {
   const uint32_t us = timer.us();
   if(us < EMIT_SLOW_MS * 1000) return;
   char what[80];
-  if(clientId == 0) snprintf(what, sizeof(what), "Socket emit %s (%u bytes) to all", this->_evt, (unsigned)strlen(this->buff));
-  else snprintf(what, sizeof(what), "Socket emit %s (%u bytes) to %lu", this->_evt, (unsigned)strlen(this->buff), (unsigned long)clientId);
+  if(clientId == 0) snprintf(what, sizeof(what), "Socket emit %s (%u bytes) to all", this->_evt, (unsigned)this->_len);
+  else snprintf(what, sizeof(what), "Socket emit %s (%u bytes) to %lu", this->_evt, (unsigned)this->_len, (unsigned long)clientId);
   logElapsed(TAG, what, us);
 }
 void JsonSockEvent::_safecat(const char *val, bool escape) {
-  size_t len = (escape ? this->calcEscapedLength(val) : strlen(val)) + strlen(this->buff);
-  if(escape) len += 2;
-  if(len >= this->buffSize) {
+  const size_t vlen = escape ? this->calcEscapedLength(val) + 2 : strlen(val);
+  if(this->_len + vlen >= this->buffSize) {
     this->_overflow = true;
     return;
   }
-  if(escape) strcat(this->buff, "\"");
-  if(escape) this->escapeString(val, &this->buff[strlen(this->buff)]);
-  else strcat(this->buff, val);
-  if(escape) strcat(this->buff, "\"");
+  this->_appendAt(val, escape, vlen);
 }
 void AsyncJsonResp::beginResponse(AsyncWebServerRequest *request, char *buff, size_t buffSize) {
   this->_request = request;
   this->buff = buff;
   this->buffSize = buffSize;
   this->buff[0] = 0x00;
+  this->_len = 0;
   this->_nocomma = true;
   this->_headersSent = false;
   this->_overflow = false;
   this->_stream = request->beginResponseStream("application/json");
 }
 void AsyncJsonResp::endResponse() {
-  if(strlen(this->buff)) this->flush();
+  if(this->_len) this->flush();
   if(this->_request && this->_stream) {
     this->_request->send(this->_stream);
   }
 }
 void AsyncJsonResp::flush() {
-  if(this->_stream && strlen(this->buff) > 0) {
-    this->_stream->print(this->buff);
+  if(this->_stream && this->_len > 0) {
+    this->_stream->write((const uint8_t *)this->buff, this->_len);
     this->buff[0] = 0x00;
+    this->_len = 0;
   }
 }
 void AsyncJsonResp::_safecat(const char *val, bool escape) {
   const size_t vlen = escape ? this->calcEscapedLength(val) + 2 : strlen(val);
-  if(vlen + strlen(this->buff) >= this->buffSize) {
+  if(vlen + this->_len >= this->buffSize) {
     this->flush();
-    if(vlen + strlen(this->buff) >= this->buffSize) {
+    if(vlen + this->_len >= this->buffSize) {
       // Bigger than the whole buffer so stream it straight out.
       if(!this->_stream) {
         this->_overflow = true;
@@ -99,8 +98,5 @@ void AsyncJsonResp::_safecat(const char *val, bool escape) {
       return;
     }
   }
-  if(escape) strcat(this->buff, "\"");
-  if(escape) this->escapeString(val, &this->buff[strlen(this->buff)]);
-  else strcat(this->buff, val);
-  if(escape) strcat(this->buff, "\"");
+  this->_appendAt(val, escape, vlen);
 }
diff --git a/src/Web.cpp b/src/Web.cpp
index f8e9414..4cc1276 100644
--- a/src/Web.cpp
+++ b/src/Web.cpp
@@ -555,7 +555,7 @@ private:
         serializeGitVersion(fmt);
         fmt.endObject();
         fmt.beginArray("rooms");
-        unitLen = strlen(unit);
+        unitLen = fmt.length();
         section = S_ROOMS;
         idx = 0;
         firstInArray = true;
@@ -577,7 +577,7 @@ private:
         fmt.beginObject();
         serializeRoom(&somfy.rooms[idx], fmt);
         fmt.endObject();
-        unitLen = pos + strlen(unit + pos);
+        unitLen = pos + fmt.length();
         idx++;
         firstInArray = false;
         return;
@@ -599,7 +599,7 @@ private:
         fmt.beginObject();
         serializeShade(&somfy.shades[idx], fmt);
         fmt.endObject();
-        unitLen = pos + strlen(unit + pos);
+        unitLen = pos + fmt.length();
         idx++;
         firstInArray = false;
         return;
@@ -622,7 +622,7 @@ private:
           fmt.beginObject();
           serializeGroupRef(g, fmt);
           fmt.beginArray("linkedShades");
-          unitLen = pos + strlen(unit + pos);
+          unitLen = pos + fmt.length();
           lsIdx = 0;
           firstInArray = false;
           firstInLSArray = true;
@@ -650,7 +650,7 @@ private:
         fmt.beginObject();
         serializeShadeRef(lshade, fmt);
         fmt.endObject();
-        unitLen = pos + strlen(unit + pos);
+        unitLen = pos + fmt.length();
         lsIdx++;
         firstInLSArray = false;
         return;
diff --git a/test/test_json/test_json.cpp b/test/test_json/test_json.cpp
index 4b21266..1dfa190 100644
--- a/test/test_json/test_json.cpp
+++ b/test/test_json/test_json.cpp
@@ -159,6 +159,42 @@ static void test_setBuffer_clears_overflow() {
   TEST_ASSERT_FALSE(fmt.overflowed());
 }
 
+/*********************************************************************
+ * Tracked length
+ ********************************************************************/
+static void test_length_matches_what_was_written() {
+  fmt.beginObject();
+  fmt.addElem("name", "Living \"Room\"\n");
+  fmt.addElem("pos", 41.5f);
+  fmt.beginArray("ids");
+  fmt.addElem((uint8_t)1);
+  fmt.addElem((int32_t)-2);
+  fmt.endArray();
+  fmt.addElem("on", true);
+  fmt.endObject();
+  TEST_ASSERT_EQUAL_UINT32(strlen(storage), fmt.length());
+  TEST_ASSERT_EQUAL_STRING("{\"name\":\"Living \\\"Room\\\"\\n\",\"pos\":41.5000,\"ids\":[1,-2],\"on\":true}", storage);
+}
+static void test_refused_append_leaves_length_alone() {
+  useBuffer(16);
+  fmt.beginObject();
+  fmt.addElem("name", "fits");
+  const size_t before = fmt.length();
+  fmt.addElem("x", "this value does not fit");
+  TEST_ASSERT_TRUE(fmt.overflowed());
+  // The comma and key still fit; the value was refused whole.
+  TEST_ASSERT_EQUAL_UINT32(strlen(storage), fmt.length());
+  TEST_ASSERT_TRUE(fmt.length() >= before);
+  TEST_ASSERT_TRUE(fmt.length() < 16);
+}
+static void test_setBuffer_resets_length() {
+  fmt.addElem("something");
+  TEST_ASSERT_TRUE(fmt.length() > 0);
+  useBuffer(BUFF_SIZE);
+  TEST_ASSERT_EQUAL_UINT32(0, fmt.length());
+  TEST_ASSERT_EQUAL_STRING("", storage);
+}
+
 int main(int argc, char **argv) {
   (void)argc;
   (void)argv;
@@ -177,5 +213,8 @@ int main(int argc, char **argv) {
   RUN_TEST(test_value_needing_one_byte_too_many_is_refused);
   RUN_TEST(test_overflow_is_flagged);
   RUN_TEST(test_setBuffer_clears_overflow);
+  RUN_TEST(test_length_matches_what_was_written);
+  RUN_TEST(test_refused_append_leaves_length_alone);
+  RUN_TEST(test_setBuffer_resets_length);
   return UNITY_END();
 }