You can run a function that will do that, and place an HTML textbox on the page to show the results. Try doing this:
<script scr="https://youSPSite/SiteAssets/jquery-2.x.x.js"></script>
<script type="text/javascript>
$(document).ready(function() {
getTotal();
});
function getTotal(){
var total = 0;
var call = jQuery.ajax({
url: _spPageContextInfo.webAbsoluteUrl + "/_api/web/lists/getByTitle('Your List Name')/items",
type: "GET",
datatype: "json",
headers: { Accept: "application/json;odata=verbose" }
});
call.done(function (data){
//console.dir(data) uncomment this to view your data and make sure your function is working in the DEV tool bar on the console
var results = data.d.results;
for (i = 0; i < results.length; i++) {
total += results[i].YourColumnToAdd;
}
var html = "<input type='text' value='" + total + "' id='txtBox' />";
$(".myTotal").append(html);
});
}
</script>
<div class="myTotal"></div>
This is a simple solution that should work to get the results you are looking for. You can play with styling and where to put it on the page. Hope this helps!
EDIT: I have adjusted the code to work in a Text file you can then use to run the code in a ContentEditorWebPart. It is best practices to use a txt file and reference it instead of putting code directly into the CEWP. CEWP will render HTML, CSS, JavaScript, C# (if security permits), and anything you can reference really. You will have to get the jQuery library and store it in your Site Assets Library and then reference it in you code (look above).
To add your web part, edit the page, and click the Add Web Part link on the page. The ContentEditor is under Content and Structure. Select the CEWP and put it on the page. Edit the CEWP and place a link to your txt file you used for code in the URL block of the CEWP properties. test the link to make sure it is correct then hit OK. Use your DEV toolbar to troubleshoot anything that might not be working just right. The txt file should be stored in your Site Assets Library.